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/rtp_sender.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <memory>
+
+#include "api/rtp_sender_interface.h"
+#include "livekit/media_stream.h"
+#include "livekit/rtc_error.h"
+#include "livekit/rtp_parameters.h"
+#include "rust/cxx.h"
+
+namespace livekit {
+class RtpSender;
+}
+#include "webrtc-sys/src/rtp_sender.rs.h"
+
+namespace livekit {
+
+// TODO(theomonnom): FrameTransformer & FrameEncryptor interface
+class RtpSender {
+ public:
+  explicit RtpSender(rtc::scoped_refptr<webrtc::RtpSenderInterface> sender);
+
+  bool set_track(std::shared_ptr<MediaStreamTrack> track) const;
+
+  std::shared_ptr<MediaStreamTrack> track() const;
+
+  uint32_t ssrc() const;
+
+  MediaType media_type() const;
+
+  rust::String id() const;
+
+  rust::Vec<rust::String> stream_ids() const;
+
+  void set_streams(const rust::Vec<rust::String>& stream_ids) const;
+
+  rust::Vec<RtpEncodingParameters> init_send_encodings() const;
+
+  RtpParameters get_parameters() const;
+
+  void set_parameters(RtpParameters params) const;
+
+  rtc::scoped_refptr<webrtc::RtpSenderInterface> get() const { return sender_; }
+
+ private:
+  rtc::scoped_refptr<webrtc::RtpSenderInterface> sender_;
+};
+
+static std::shared_ptr<RtpSender> _shared_rtp_sender() {
+  return nullptr;  // Ignore
+}
+}  // namespace livekit