Send side Simulcast/Track encodings

[haaspors]

Most of this is ported from different patches in pion related to send side simulcast.

First and foremost, a track can now be created with a specific RID, and RtpSender::add_encoding() is used for adding track encodings. Also, MID and RID RTP header extensions are automatically added when sending RTP/Samples on local tracks. All of this would only work if MID and RID header extensions are enabled/registered on media engine.

[codecov]

Codecov Report

Attention: Patch coverage is 66.50718% with 140 lines in your changes missing coverage. Please review.

Project coverage is 61.80%. Comparing base (ae93e81) to head (539532b).
Report is 1 commits behind head on master.

Files	Patch %	Lines
.../src/rtp_transceiver/rtp_sender/rtp_sender_test.rs	69.16%	5 Missing and 65 partials ⚠️
...tc/src/peer_connection/peer_connection_internal.rs	28.81%	36 Missing and 6 partials ⚠️
webrtc/src/peer_connection/peer_connection_test.rs	77.52%	2 Missing and 18 partials ⚠️
...tc/src/track/track_local/track_local_static_rtp.rs	75.00%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
+ Coverage   61.57%   61.80%   +0.22%     
==========================================
  Files         532      532              
  Lines       48970    49331     +361     
  Branches    12416    12500      +84     
==========================================
+ Hits        30155    30487     +332     
+ Misses       9582     9507      -75     
- Partials     9233     9337     +104     

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