sdp: Handle level-asymmetry-allowed for H264 streams

sdp: Handle level-asymmetry-allowed for H264 streams

The ["level-asymmetry-allowed"] field states that the peer wants the
profile specified in the "profile-level-id" fields but doesn't care
about the level. To express this in GStreamer caps term, we add a
"profile" field in the caps, which reuses the usual "profile" semantics
for H.264 streams and, and remove "profile-level-id" and
"level-asymmetry-allowed" fields.

["level-asymmetry-allowed"]: https://www.iana.org/assignments/media-types/video/H264

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1410>