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

webrtc-sys/include/livekit/helper.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "rust/cxx.h"
+
+namespace livekit {
+class MediaStream;
+class AudioTrack;
+class VideoTrack;
+class Candidate;
+class RtpSender;
+class RtpReceiver;
+class RtpTransceiver;
+}  // namespace livekit
+#include "webrtc-sys/src/helper.rs.h"
+
+namespace livekit {
+
+// Impl not needed
+static rust::Vec<MediaStreamPtr> _vec_media_stream_ptr() {
+  throw;
+}
+static rust::Vec<CandidatePtr> _vec_candidate_ptr() {
+  throw;
+}
+static rust::Vec<AudioTrackPtr> _vec_audio_track_ptr() {
+  throw;
+}
+static rust::Vec<VideoTrackPtr> _vec_video_track_ptr() {
+  throw;
+}
+static rust::Vec<RtpSenderPtr> _vec_rtp_sender_ptr() {
+  throw;
+}
+static rust::Vec<RtpReceiverPtr> _vec_rtp_receiver_ptr() {
+  throw;
+}
+static rust::Vec<RtpTransceiverPtr> _vec_rtp_transceiver_ptr() {
+  throw;
+}
+
+}  // namespace livekit