Pass all codecs instead of selected one to the rtp_receiver and rtp_sender

[mickel8]

This PR passes all codecs that were negotiated to the RTP sender and RTP receiver. Because in our case these codecs are always the same i.e. (transceiver.codecs==sender.codecs==receiver.codecs), they are not publicly exposed in RTPSender and RTPReceiver structs to not create confusion or suggest that they may differ.

The behavior of RTP sender and receiver remains the same i.e. they always pick the first codec and use it for sending or expect it when receiving RTP packets.

In the next PR, I am going to allow to set codec that should be used for sending.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.67%. Comparing base (f87341e) to head (60dd117).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
- Coverage   84.72%   84.67%   -0.05%     
==========================================
  Files          49       49              
  Lines        2507     2512       +5     
==========================================
+ Hits         2124     2127       +3     
- Misses        383      385       +2     

Files with missing lines	Coverage Δ
lib/ex_webrtc/rtp_receiver.ex	78.04% <100.00%> (+1.12%)	⬆️
lib/ex_webrtc/rtp_sender.ex	93.40% <100.00%> (+0.54%)	⬆️
lib/ex_webrtc/rtp_transceiver.ex	90.36% <100.00%> (-0.40%)	⬇️
lib/ex_webrtc/utils.ex	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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