sdp: introduce MCD::AttributeLevel for session/media-level attrs

Generalizes the enum used to track whether an attribute is signalled at
session level or media level. The previous enum was specific to
extmap-allow-mixed (ExtmapAllowMixed { kNo, kSession, kMedia }); rename
it to AttributeLevel { kNone, kSession, kMedia } so it can be reused.

Migrates extmap-allow-mixed to the new type:
  ExtmapAllowMixed              -> AttributeLevel
  kNo                           -> kNone
  set_extmap_allow_mixed_enum() -> set_extmap_allow_mixed_level()
  extmap_allow_mixed_enum()     -> extmap_allow_mixed_level()

No (intended) behavior change.
A follow-up CL will use AttributeLevel for cryptex.

Bug: webrtc:455813732
Change-Id: If68ad7e32dbfea61f3e122b719488d18ceb88290
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470580
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#47656}