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/objc_video_factory.h

@@ -1,7 +1,7 @@
-#ifndef OBJC_VIDEO_FACTORY_H
-#define OBJC_VIDEO_FACTORY_H
+#pragma once
 
 #include <memory>
+
 #include "api/video_codecs/video_decoder_factory.h"
 #include "api/video_codecs/video_encoder_factory.h"
 
@@ -11,5 +11,3 @@ std::unique_ptr<webrtc::VideoEncoderFactory> CreateObjCVideoEncoderFactory();
 std::unique_ptr<webrtc::VideoDecoderFactory> CreateObjCVideoDecoderFactory();
 
 }  // namespace livekit
-
-#endif // OBJC_VIDEO_FACTORY_H