webrtcsink: add define-encoder-bitrates signal
When congestion control is used for a session with multiple encoders, the default implementation simply divides the overall bitrate equally between encoders.
This is not always desirable, and this patch exposes a new signal that users can register to, with two arguments:
- The overall bitrate to allocate
- A structure with an encoder.stream_name -> bitrate mapping
Handlers should return a similar structure with a custom mapping.
An example is also provided.