feat: send encoded frames through RTP sender injectors

[devopvoid]

Wraps WebRTC's encoded frame injectors (new in M155), which send frames that are already encoded in place of a sender's track.

• RTCRtpSender.createEncodedVideoFrameInjector(...) / createEncodedAudioFrameInjector(...), with observers for key frame requests and bitrate changes, called on the callback thread from fix: call network thread observers on a callback thread #364.
• RTCPeerConnection.addTransceiver(MediaType, init) for senders without a track.
• WebRTC's frame factory is outside its default build, so the CMake build now builds and links a list of extra targets; an existing WebRTC install rebuilds once.
• A section on injectors in the encoded transforms guide.

Stacked on #364.