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, 47 insertions, 0 deletions

diff --git a/src/zen/bench.cpp b/src/zen/bench.cpp
index 614454ed5..2332ce1b8 100644
--- a/src/zen/bench.cpp
+++ b/src/zen/bench.cpp
@@ -119,6 +119,53 @@ EmptyStandByList()
 
 }  // namespace zen::bench::util
 
+#elif ZEN_PLATFORM_LINUX
+
+#	include <fcntl.h>
+#	include <unistd.h>
+
+namespace zen::bench::util {
+
+void
+EmptyStandByList()
+{
+	sync();
+
+	int Fd = open("/proc/sys/vm/drop_caches", O_WRONLY);
+	if (Fd < 0)
+	{
+		throw std::runtime_error("Failed to open /proc/sys/vm/drop_caches (are you running as root?)");
+	}
+
+	if (write(Fd, "3", 1) != 1)
+	{
+		close(Fd);
+		throw std::runtime_error("Failed to write to /proc/sys/vm/drop_caches");
+	}
+
+	close(Fd);
+}
+
+}  // namespace zen::bench::util
+
+#elif ZEN_PLATFORM_MAC
+
+#	include <cstdlib>
+
+namespace zen::bench::util {
+
+void
+EmptyStandByList()
+{
+	int Result = system("/usr/sbin/purge");
+	if (Result != 0)
+	{
+		throw std::runtime_error("Failed to run /usr/sbin/purge (are you running as root?)");
+	}
+}
+
+}  // namespace zen::bench::util
+
 #else
 
 namespace zen::bench::util {