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/video_encoder_factory.h

@@ -1,8 +1,7 @@
-#ifndef VIDEO_ENCODER_FACTORY_H
-#define VIDEO_ENCODER_FACTORY_H
+#pragma once
 
-#include "api/video_codecs/video_encoder_factory.h"
 #include "api/video_codecs/video_encoder.h"
+#include "api/video_codecs/video_encoder_factory.h"
 
 namespace livekit {
 class VideoEncoderFactory : public webrtc::VideoEncoderFactory {
@@ -18,5 +17,3 @@ class VideoEncoderFactory : public webrtc::VideoEncoderFactory {
   std::vector<std::unique_ptr<webrtc::VideoEncoderFactory>> factories_;
 };
 }  // namespace livekit
-
-#endif  // VIDEO_ENCODER_FACTORY_H