RtpTransceiver bindings & requirements for publishing tracks (#39)

- RtpSender
- RtpReceiver
- RtpTransceiver
- RtpParameters
- VideoFrameBuilder
- Interior mutability on c++ side
- Cleanup bindings ( Use #pragma once instead of include guards )
     - webrtc-sys/src/* headers are now used in only one place 
     - Avoid confusion between generated headers and our headers
- AdaptedVideoTrackSource
- Cleanup the workaround with unsupported Vec<Shared<T>>
    - Put everything inside one file

livekit-ffi/src/server/mod.rs

@@ -91,7 +91,7 @@ impl FFIServer {
 
     pub async fn close(&self) {
         // Close all rooms
-        for (k, (handle, shutdown_tx)) in self.rooms.write().drain() {
+        for (_, (handle, shutdown_tx)) in self.rooms.write().drain() {
             let _ = shutdown_tx.send(());
             let _ = handle.await;
         }

livekit-ffi/src/server/room.rs

@@ -116,4 +116,3 @@ fn on_video_frame(server: &'static FFIServer, track_sid: TrackSid) -> OnFrameHan
         );
     })
 }
-//