Parsing simulcast according to:
https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast-13#section-5.1
Created SdpSerializer for making serialized components more testable.
Simulcast functionality is still not accessible to users.
Bug: webrtc:10055
Change-Id: Ia6e4cef756cb954521dd19e22911f8eb6498880e
Reviewed-on: https://webrtc-review.googlesource.com/c/112160
Commit-Queue: Amit Hilbuch <amithi@webrtc.org>
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25883}
