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, 30 insertions, 13 deletions

diff --git a/src/zenhttp/servers/wsasio.h b/src/zenhttp/servers/wsasio.h
index e8bb3b1d2..64602ee46 100644
--- a/src/zenhttp/servers/wsasio.h
+++ b/src/zenhttp/servers/wsasio.h
@@ -8,6 +8,12 @@
 
 ZEN_THIRD_PARTY_INCLUDES_START
 #include <asio.hpp>
+#if defined(ASIO_HAS_LOCAL_SOCKETS)
+#	include <asio/local/stream_protocol.hpp>
+#endif
+#if ZEN_USE_OPENSSL
+#	include <asio/ssl.hpp>
+#endif
 ZEN_THIRD_PARTY_INCLUDES_END
 
 #include <deque>
@@ -21,22 +27,23 @@ class HttpServer;
 namespace zen::asio_http {
 
 /**
- * WebSocket connection over an ASIO TCP socket
+ * WebSocket connection over an ASIO stream socket
  *
- * Owns the TCP socket (moved from HttpServerConnection after the 101 handshake)
+ * Templated on SocketType to support both TCP and Unix domain sockets.
+ * Owns the socket (moved from HttpServerConnection after the 101 handshake)
  * and runs an async read/write loop to exchange WebSocket frames.
  *
  * Lifetime is managed solely through intrusive reference counting (RefCounted).
- * The async read/write callbacks capture Ref<WsAsioConnection> to keep the
- * connection alive for the duration of the async operation.  The service layer
- * also holds a Ref<WebSocketConnection>.
+ * The async read/write callbacks capture Ref<> to keep the connection alive
+ * for the duration of the async operation.  The service layer also holds a
+ * Ref<WebSocketConnection>.
  */
-
-class WsAsioConnection : public WebSocketConnection
+template<typename SocketType>
+class WsAsioConnectionT : public WebSocketConnection
 {
 public:
-	WsAsioConnection(std::unique_ptr<asio::ip::tcp::socket> Socket, IWebSocketHandler& Handler, HttpServer* Server);
-	~WsAsioConnection() override;
+	WsAsioConnectionT(std::unique_ptr<SocketType> Socket, IWebSocketHandler& Handler, HttpServer* Server);
+	~WsAsioConnectionT() override;
 
 	/**
 	 * Start the async read loop. Must be called once after construction
@@ -61,10 +68,10 @@ private:
 
 	void DoClose(uint16_t Code, std::string_view Reason);
 
-	std::unique_ptr<asio::ip::tcp::socket> m_Socket;
-	IWebSocketHandler&					   m_Handler;
-	zen::HttpServer*					   m_HttpServer;
-	asio::streambuf						   m_ReadBuffer;
+	std::unique_ptr<SocketType> m_Socket;
+	IWebSocketHandler&			m_Handler;
+	zen::HttpServer*			m_HttpServer;
+	asio::streambuf				m_ReadBuffer;
 
 	RwLock							 m_WriteLock;
 	std::deque<std::vector<uint8_t>> m_WriteQueue;
@@ -74,4 +81,14 @@ private:
 	std::atomic<bool> m_CloseSent{false};
 };
 
+using WsAsioConnection = WsAsioConnectionT<asio::ip::tcp::socket>;
+
+#if defined(ASIO_HAS_LOCAL_SOCKETS)
+using WsAsioUnixConnection = WsAsioConnectionT<asio::local::stream_protocol::socket>;
+#endif
+
+#if ZEN_USE_OPENSSL
+using WsAsioSslConnection = WsAsioConnectionT<asio::ssl::stream<asio::ip::tcp::socket>>;
+#endif
+
 }  // namespace zen::asio_http