Give WebSocketClient the PemMemory client certificate constructor SSLClient has

Adds ws::WebSocketClient::PemMemory and a constructor overload that
installs an in-memory client certificate on the TLS context, enabling
mutual TLS for wss:// connections. The certificate is silently ignored
for ws:// URLs, consistent with the existing TLS-only setters such as
set_ca_cert_path().

Part of the interface alignment discussed in #2531.

httplib.h

@@ -4300,6 +4300,16 @@ public:
   void set_hostname_addr_map(std::map<std::string, std::string> addr_map);
 
 #ifdef CPPHTTPLIB_SSL_ENABLED
+  struct PemMemory {
+    const char *cert_pem;
+    size_t cert_pem_len;
+    const char *key_pem;
+    size_t key_pem_len;
+    const char *private_key_password;
+  };
+  explicit WebSocketClient(const std::string &scheme_host_port_path,
+                           const PemMemory &pem, const Headers &headers = {});
+
   void set_ca_cert_path(const std::string &ca_cert_file_path,
                         const std::string &ca_cert_dir_path = std::string());
   void set_ca_cert_store(tls::ca_store_t store);
@@ -21289,6 +21299,24 @@ inline WebSocketClient::WebSocketClient(
   }
 }
 
+#ifdef CPPHTTPLIB_SSL_ENABLED
+inline WebSocketClient::WebSocketClient(
+    const std::string &scheme_host_port_path, const PemMemory &pem,
+    const Headers &headers)
+    : WebSocketClient(scheme_host_port_path, headers) {
+  // For ws:// URLs the client certificate is silently ignored, consistent
+  // with the TLS-only setters such as set_ca_cert_path().
+  if (is_valid_ && is_ssl_ && pem.cert_pem && pem.key_pem) {
+    if (!tls::set_client_cert_pem(tls_ctx_, pem.cert_pem, pem.key_pem,
+                                  pem.private_key_password)) {
+      tls::free_context(tls_ctx_);
+      tls_ctx_ = nullptr;
+      is_valid_ = false;
+    }
+  }
+}
+#endif
+
 inline WebSocketClient::~WebSocketClient() {
   shutdown_and_close();
 #ifdef CPPHTTPLIB_SSL_ENABLED