webrtcsink: Add support for encoded streams

• webrtc: Unify the Codec structure between sink and source

• webrtcsink: Add support for pre encoded streams
  This is a first step where we try to replicate encoding conditions from the input stream into the discovery pipeline. A second patch will implement using input buffers in the discovery pipelines.

• webrtcsink: Move discovery to using input buffers directly
  Instead of trying to replicate buffers that webrtcsink is getting as input with testsrc, directly run discovery based on the real buffers. This way we are sure we work with the exact right stream type and we don't need encoders to support encoding streams inputs.

  We use the same logic for both encoded and raw input to avoid having several code paths and makes it all more correct in any case.

requested review from @meh

added 4 commits

• b6e4c001 - webrtc: Minor documentation enhancement
• 6e759e0d - webrtc: Move make_element to the utils
• 95b99da6 - webrtc: Unify the Codec structure between sink and source
• efb16111 - webrtcsink: Add support for pre encoded streams

Compare with previous version

changed title from webrtcsink: Add support for pre encoded streams to webrtcsink: Add support for encoded streams

Not completely onboard with the Codecs design for now, I'll look at the final commit tomorrow :)

By the way the commit message for the final commit mentions that it is a "first step" and that a follow up commit will implement the second step, did you fix both commits up? If so, can you update the message? :)

mentioned in merge request !1176 (merged)

Disclaimer, I didn't read through the patch, is there a plan to expose the internal feedback from the rate controller so that one can use it externally ? (e.g. if we create SVC controllers)