Add option to disable picture loss indication support in SFU.

[marcusspangenberg]

In broadcast use cases, key frames and picture loss indication (PLI) feedback from participants needs to be handled differently than in a conferencing use case.

• Key frame interval from the encoder must be short (seconds)
• PLI feedback from viewers must be ignored. Since a PLI will cause the encoder to generate a new key frame, causing bandwidth spikes that will affect all viewers of a broadcast. If the key frame interval is short enough, the viewers will get a key frame eventually anyways.