HttpClient using libcurl, Unix Sockets for HTTP. HTTPS support (#770)

The main goal of this change is to eliminate the cpr back-end altogether and replace it with the curl implementation. I would expect to drop cpr as soon as we feel happy with the libcurl back-end. That would leave us with a direct dependency on libcurl only, and cpr can be eliminated as a dependency.

### HttpClient Backend Overhaul
- Implemented a new **libcurl-based HttpClient** backend (`httpclientcurl.cpp`, ~2000 lines)
  as an alternative to the cpr-based one
- Made HttpClient backend **configurable at runtime** via constructor arguments
  and `-httpclient=...` CLI option (for zen, zenserver, and tests)
- Extended HttpClient test suite to cover multipart/content-range scenarios

### Unix Domain Socket Support
- Added Unix domain socket support to **httpasio** (server side)
- Added Unix domain socket support to **HttpClient**
- Added Unix domain socket support to **HttpWsClient** (WebSocket client)
- Templatized `HttpServerConnectionT<SocketType>` and `WsAsioConnectionT<SocketType>`
  to handle TCP, Unix, and SSL sockets uniformly via `if constexpr` dispatch

### HTTPS Support
- Added **preliminary HTTPS support to httpasio** (for Mac/Linux via OpenSSL)
- Added **basic HTTPS support for http.sys** (Windows)
- Implemented HTTPS test for httpasio
- Split `InitializeServer` into smaller sub-functions for http.sys

### Other Notable Changes
- Improved **zenhttp-test stability** with dynamic port allocation
- Enhanced port retry logic in http.sys (handles ERROR_ACCESS_DENIED)
- Fatal signal/exception handlers for backtrace generation in tests
- Added `zen bench http` subcommand to exercise network + HTTP client/server communication stack

1 files changed, 135 insertions, 0 deletions

diff --git a/src/zenhttp/clients/httpclientcurl.h b/src/zenhttp/clients/httpclientcurl.h
new file mode 100644
index 000000000..2a49ff308
--- /dev/null
+++ b/src/zenhttp/clients/httpclientcurl.h
@@ -0,0 +1,135 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+#include "httpclientcommon.h"
+
+#include <zencore/logging.h>
+#include <zenhttp/httpclient.h>
+
+ZEN_THIRD_PARTY_INCLUDES_START
+#include <curl/curl.h>
+ZEN_THIRD_PARTY_INCLUDES_END
+
+namespace zen {
+
+class CurlHttpClient : public HttpClientBase
+{
+public:
+	CurlHttpClient(std::string_view BaseUri, const HttpClientSettings& ConnectionSettings, std::function<bool()>&& CheckIfAbortFunction);
+	~CurlHttpClient();
+
+	// HttpClientBase
+
+	[[nodiscard]] virtual Response Put(std::string_view Url, const IoBuffer& Payload, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Put(std::string_view Url, const KeyValueMap& Parameters = {}) override;
+	[[nodiscard]] virtual Response Get(std::string_view	  Url,
+									   const KeyValueMap& AdditionalHeader = {},
+									   const KeyValueMap& Parameters	   = {}) override;
+	[[nodiscard]] virtual Response Head(std::string_view Url, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Delete(std::string_view Url, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view   Url,
+										const KeyValueMap& AdditionalHeader = {},
+										const KeyValueMap& Parameters		= {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view Url, const IoBuffer& Payload, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view   Url,
+										const IoBuffer&	   Payload,
+										ZenContentType	   ContentType,
+										const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view Url, CbObject Payload, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view Url, CbPackage Payload, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Post(std::string_view	   Url,
+										const CompositeBuffer& Payload,
+										ZenContentType		   ContentType,
+										const KeyValueMap&	   AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Upload(std::string_view Url, const IoBuffer& Payload, const KeyValueMap& AdditionalHeader = {}) override;
+	[[nodiscard]] virtual Response Upload(std::string_view		 Url,
+										  const CompositeBuffer& Payload,
+										  ZenContentType		 ContentType,
+										  const KeyValueMap&	 AdditionalHeader = {}) override;
+
+	[[nodiscard]] virtual Response Download(std::string_view			 Url,
+											const std::filesystem::path& TempFolderPath,
+											const KeyValueMap&			 AdditionalHeader = {}) override;
+
+	[[nodiscard]] virtual Response TransactPackage(std::string_view	  Url,
+												   CbPackage		  Package,
+												   const KeyValueMap& AdditionalHeader = {}) override;
+
+private:
+	struct CurlResult
+	{
+		long											 StatusCode = 0;
+		std::string										 Body;
+		std::vector<std::pair<std::string, std::string>> Headers;
+		double											 ElapsedSeconds	 = 0;
+		int64_t											 UploadedBytes	 = 0;
+		int64_t											 DownloadedBytes = 0;
+		CURLcode										 ErrorCode		 = CURLE_OK;
+		std::string										 ErrorMessage;
+	};
+
+	struct Session
+	{
+		Session(CurlHttpClient* InOuter, CURL* InHandle) : Outer(InOuter), Handle(InHandle) {}
+		~Session() { Outer->ReleaseSession(Handle); }
+
+		CURL* Get() const { return Handle; }
+
+		CurlResult Perform();
+
+		LoggerRef Log() { return Outer->Log(); }
+
+	private:
+		CurlHttpClient* Outer;
+		CURL*			Handle;
+
+		Session(Session&&) = delete;
+		Session& operator=(Session&&) = delete;
+	};
+
+	Session AllocSession(std::string_view					  BaseUrl,
+						 std::string_view					  Url,
+						 const HttpClientSettings&			  ConnectionSettings,
+						 const KeyValueMap&					  AdditionalHeader,
+						 const KeyValueMap&					  Parameters,
+						 std::string_view					  SessionId,
+						 std::optional<HttpClientAccessToken> AccessToken);
+
+	RwLock			   m_SessionLock;
+	std::vector<CURL*> m_Sessions;
+
+	void ReleaseSession(CURL* Handle);
+
+	struct RetryResult
+	{
+		CurlResult Result;
+	};
+
+	CurlResult DoWithRetry(std::string_view							 SessionId,
+						   std::function<CurlResult()>&&			 Func,
+						   std::unique_ptr<detail::TempPayloadFile>& PayloadFile);
+	CurlResult DoWithRetry(
+		std::string_view				   SessionId,
+		std::function<CurlResult()>&&	   Func,
+		std::function<bool(CurlResult&)>&& Validate = [](CurlResult&) { return true; });
+
+	bool ValidatePayload(CurlResult& Result, std::unique_ptr<detail::TempPayloadFile>& PayloadFile);
+
+	static bool ShouldRetry(const CurlResult& Result);
+
+	bool ShouldLogErrorCode(HttpResponseCode ResponseCode) const;
+
+	HttpClient::Response CommonResponse(std::string_view									   SessionId,
+										CurlResult&&										   Result,
+										IoBuffer&&											   Payload,
+										std::vector<HttpClient::Response::MultipartBoundary>&& BoundaryPositions = {});
+
+	HttpClient::Response ResponseWithPayload(std::string_view										SessionId,
+											 CurlResult&&											Result,
+											 const HttpResponseCode									WorkResponseCode,
+											 IoBuffer&&												Payload,
+											 std::vector<HttpClient::Response::MultipartBoundary>&& BoundaryPositions);
+};
+
+}  // namespace zen