webrtcsink now implements a generic control data channel

webrtcsink already supported instantiating a data channel for the sole purpose of carrying navigation events from the consumer to the producer, it can also now create a generic control data channel through which the consumer can send JSON requests in the form:

{
    "id": identifier used in the response message,
    "mid": optional media identifier the request applies to,
    "request": {
        "type": currently "navigationEvent" and "customUpstreamEvent" are supported,
        "type-specific-field": ...
    }
}

The producer will reply with such messages:

{
  "id": identifier of the request,
  "error": optional error message, successful if not set
}

The example frontend was also updated with a text area for sending any arbitrary request.

The use case for this work was to make it possible for a consumer to control the mix matrix used for the audio stream, with such a pipeline running on the producer side:

gst-launch-1.0 audiotestsrc ! audioconvert ! webrtcsink enable-control-data-channel=true

As audioconvert now supports setting a mix matrix through a custom upstream event, the consumer can simply input the following text in the request field of the frontend to reverse the channels of a stereo audio stream:

{
  "type": "customUpstreamEvent",
  "structureName": "GstRequestAudioMixMatrix",
  "structure": {
    "matrix": [[0.0, 1.0], [1.0, 0.0]]
  }
}