Add ssrc attributes to the SDP

[mickel8]

Some implementations still rely on ssrc attributes in SDP offer/answer when demuxing incoming RTP packets (e.g. Pion). While including a=ssrc attribute in the SDP is obsoleted and we should demux by mid, this is still correct and supported by web browsers.

This PR adds ssrc attributes to the SDP offer/answer when a transceiver is sendonly or sendrecv.

A couple of notes:

• in theory, if we add a=ssrc:1234 attribute, it should also always has cname attr i.e. a=ssrc:1234 cname:qW34eced
• we don't use cnames so we don't do this. Instead we always add msid attribute as we already have it
• we stop sending app_data in msid according to what JSEP says. This app_data does not make any sense anyway as its value is MediaStreamTrack.id, which can change without renegotiation (e.g. when calling rtpsender.replaceTrack)

[codecov]

Codecov Report

Attention: Patch coverage is 87.27273% with 7 lines in your changes missing coverage. Please review.

Project coverage is 84.78%. Comparing base (e0ff731) to head (008b334).

Files with missing lines	Patch %	Lines
lib/ex_webrtc/rtp_sender.ex	70.83%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage   84.75%   84.78%   +0.02%     
==========================================
  Files          49       49              
  Lines        2479     2510      +31     
==========================================
+ Hits         2101     2128      +27     
- Misses        378      382       +4     

Files with missing lines	Coverage Δ
lib/ex_webrtc/peer_connection.ex	86.39% <100.00%> (+0.22%)	⬆️
lib/ex_webrtc/peer_connection/configuration.ex	94.89% <ø> (ø)
lib/ex_webrtc/rtp_transceiver.ex	90.75% <100.00%> (-0.21%)	⬇️
lib/ex_webrtc/rtp_sender.ex	85.88% <70.83%> (-7.57%)	⬇️

... and 3 files with indirect coverage changes

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