Add experiment to use ::recvmsg to receive packets on posix systems

Using ::recvmsg ensure packet timestamp can then be read directly when reading the buffer
instead of a separate system call and should also work on Ios/Mac.

The same experiment field trial flag will be "WebRTC-SCM-Timestamp/enabled/" and is also planned to be used for fixing webrtc:14066

Bug: webrtc:5773, webrtc:14066
Change-Id: I8a3749e87c686aa18fcee947472c1b602a0f63c8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/279280
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38585}

rtc_base/physical_socket_server.h

@@ -190,6 +190,11 @@ class PhysicalSocket : public Socket, public sigslot::has_slots<> {
                        const struct sockaddr* dest_addr,
                        socklen_t addrlen);
 
+  int DoReadFromSocket(void* buffer,
+                       size_t length,
+                       SocketAddress* out_addr,
+                       int64_t* timestamp);
+
   void OnResolveResult(AsyncResolverInterface* resolver);
 
   void UpdateLastError();
@@ -216,6 +221,7 @@ class PhysicalSocket : public Socket, public sigslot::has_slots<> {
 #endif
 
  private:
+  const bool read_scm_timestamp_experiment_;
   uint8_t enabled_events_ = 0;
 };