webrtc: add raw payload support

Depends on

Only the last commit is specific to this MR, the other commits implement other features which serve as a basis for this work and come from the following MRs:

• !1463 (merged): this is the base infrastructure for the precise example MR stack. It aims at providing the same features & convenience as the RTSP based rtp-rapid-sync-example for WebRTC. It helped improve the WebRTC C stack with support for intra CNAME synchronization & RFC 6051 in-band NTP-64 timestamps for rapid synchronization. The example also allows spawning an arbitrary number of audio and/or video streams.
• !1500 (merged): adds RFC 7273 clock signalling to webrtcsink.

Raw payload support

This MR adds support for raw payloads such as L24 audio to webrtcsink & webrtcsrc.

Most changes take place within the Codec helper structure:

• A Codec can now advertise a depayloader. This also ensures that a format not only can be decoded when necessary, but it can also be depayloaded in the first place.
• It is possible to declare raw Codecs, meaning that their caps are compatible with a payloader and a depayloader without the need for an encoder and decoder.
• Previous accessor has_decoder was renamed as can_be_received to account for codecs which can be handled by an available depayloader with or without the need for a decoder.
• New codecs were added for the following formats:
  • L24, L16, L8 audio.
  • RAW video.

The webrtc-precise-sync examples were updated to demonstrate streaming of raw audio or video.