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, 9 insertions, 0 deletions

diff --git a/src/zenserver/zenserver.cpp b/src/zenserver/zenserver.cpp
index 88b85d7d9..49ae1b6ff 100644
--- a/src/zenserver/zenserver.cpp
+++ b/src/zenserver/zenserver.cpp
@@ -23,6 +23,7 @@
 #include <zencore/timer.h>
 #include <zencore/trace.h>
 #include <zencore/workthreadpool.h>
+#include <zenhttp/httpclient.h>
 #include <zenhttp/httpserver.h>
 #include <zenhttp/security/passwordsecurityfilter.h>
 #include <zentelemetry/otlptrace.h>
@@ -146,6 +147,14 @@ ZenServerBase::Initialize(const ZenServerConfig& ServerOptions, ZenServerState::
 
 	EnqueueSigIntTimer();
 
+	// Configure HTTP client back-end
+
+	const std::string HttpClientBackend = ToLower(ServerOptions.HttpClient.Backend);
+	zen::SetDefaultHttpClientBackend(HttpClientBackend);
+	ZEN_INFO("Using '{}' as HTTP client backend", HttpClientBackend);
+
+	// Initialize HTTP server
+
 	m_Http				  = CreateHttpServer(ServerOptions.HttpConfig);
 	int EffectiveBasePort = m_Http->Initialize(ServerOptions.BasePort, ServerOptions.DataDir);
 	if (EffectiveBasePort == 0)