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.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1792>
