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, 54 insertions, 0 deletions

diff --git a/src/zenhttp/servers/asio_socket_traits.h b/src/zenhttp/servers/asio_socket_traits.h
new file mode 100644
index 000000000..25aeaa24e
--- /dev/null
+++ b/src/zenhttp/servers/asio_socket_traits.h
@@ -0,0 +1,54 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#pragma once
+
+ZEN_THIRD_PARTY_INCLUDES_START
+#include <asio.hpp>
+#if ZEN_USE_OPENSSL
+#	include <asio/ssl.hpp>
+#endif
+ZEN_THIRD_PARTY_INCLUDES_END
+
+namespace zen::asio_http {
+
+/**
+ * Traits for abstracting socket shutdown/close across plain TCP, Unix domain, and SSL sockets.
+ * SSL sockets need lowest_layer() access and have different shutdown semantics.
+ */
+template<typename SocketType>
+struct SocketTraits
+{
+	/// SSL sockets cannot use zero-copy file send (TransmitFile/sendfile) because
+	/// those bypass the encryption layer. This flag lets templated code fall back
+	/// to reading-into-memory for SSL connections.
+	static constexpr bool IsSslSocket = false;
+
+	static void ShutdownReceive(SocketType& S, std::error_code& Ec) { S.shutdown(asio::socket_base::shutdown_receive, Ec); }
+
+	static void ShutdownBoth(SocketType& S, std::error_code& Ec) { S.shutdown(asio::socket_base::shutdown_both, Ec); }
+
+	static void Close(SocketType& S, std::error_code& Ec) { S.close(Ec); }
+};
+
+#if ZEN_USE_OPENSSL
+using SslSocket = asio::ssl::stream<asio::ip::tcp::socket>;
+
+template<>
+struct SocketTraits<SslSocket>
+{
+	static constexpr bool IsSslSocket = true;
+
+	static void ShutdownReceive(SslSocket& S, std::error_code& Ec) { S.lowest_layer().shutdown(asio::socket_base::shutdown_receive, Ec); }
+
+	static void ShutdownBoth(SslSocket& S, std::error_code& Ec)
+	{
+		// Best-effort SSL close_notify, then TCP shutdown
+		S.shutdown(Ec);
+		S.lowest_layer().shutdown(asio::socket_base::shutdown_both, Ec);
+	}
+
+	static void Close(SslSocket& S, std::error_code& Ec) { S.lowest_layer().close(Ec); }
+};
+#endif
+
+}  // namespace zen::asio_http