Must join all XxxConsumer classes into a single one

[ibc]

Details

• In C++ we currently have a parent abstract Consumer class and then SimpleConsumer, SimulcastConsumer, SvcConsumer and PipeConsumer that inherit from Consumer.
• Those classes are intended to handle different producing scenarios and associated codecs.
• For example a video transmission could use VP8 in single stream mode so it would use SingleConsumer.
• Or with N streams in simulcast mode so it would use SimulcastConsumer.

Problems

• Those classes do not represent real scenarios.
• For example, a producer may perfectly use VP8 with a single stream (so this is not simulcast) and temporal layers. We currently manage it in SimulcastConsumer since it's the one ready for VP8 codec, but this is SVC.
• VP9 is supposed to be SVC, however in libwebrtc now it's possible to produce real simulcast using VP9 (probably also with temporal layers): https://groups.google.com/g/discuss-webrtc/c/-QQ3pxrl-fw

Proposal

• Have a single Consumer class ready to deal with all use cases and all supported codecs.
• This would also help in the future when we implement the ability to switch from a producer to another in the same consumer.
• We should also reconsider those producer.type and consumer.type ("single", "simulcast", "svc" and "pipe") since they don't make any sense. However this would introduce breaking changes. Let's see how to do it.