Pre-encoded video publish adds passthrough encoder

Features

• Add a pre-encoded video publish path: a passthrough video encoder and encoded video frame buffer in webrtc-sys, and EncodedVideoFrame/EncodedVideoCodec/EncodedFrameType publish APIs with a VideoEncoderBackend::PreEncoded backend in libwebrtc. WebRTC rate-control targets and keyframe requests are forwarded to encoded sources, and pre-encoded AV1 and H265 access units are validated on ingest.