More std::string usage updates in pc/ using absl::string_view

Reduce std::string usage in SessionDescription and related classes by
migrating from std::string to absl::string_view for various identifiers
and accessors.

Key changes:
- Updated SessionDescription, ContentInfo, ContentGroup, and
  MediaContentDescription APIs to use absl::string_view where
  appropriate.
- Updated call sites in pc/ and tests to match.
- use flat_map and flat_set where before we used std::map and std::set
  together with std::string. The flat versions are more compatible with
  absl::string_view and also more efficient.
- Update dependencies.

Bug: webrtc:42223790
Change-Id: I5eb9ae08f74aaee946eec2a437b64d6090b155ca
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/433482
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46488}