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/candidate.h

@@ -2,13 +2,17 @@
 // Created by Théo Monnom on 01/09/2022.
 //
 
-#ifndef CLIENT_SDK_NATIVE_CANDIDATE_H
-#define CLIENT_SDK_NATIVE_CANDIDATE_H
+#pragma once
 
 #include <memory>
 
 #include "api/candidate.h"
 
+namespace livekit {
+class Candidate;
+}
+#include "webrtc-sys/src/candidate.rs.h"
+
 // cricket::Candidate
 namespace livekit {
 
@@ -20,10 +24,8 @@ class Candidate {
   cricket::Candidate candidate_;
 };
 
-static std::unique_ptr<Candidate> _unique_candidate() {
+static std::shared_ptr<Candidate> _shared_candidate() {
   return nullptr;
 }
 
 }  // namespace livekit
-
-#endif  // CLIENT_SDK_NATIVE_CANDIDATE_H