Use StrongAlias for RTP header extension identification.

This change introduces RtpHeaderExtensionId as a StrongAlias to
improve type safety for RTP header extension IDs. The type is used
throughout the RTP stack.

The code previous to this change has many different places where it
defines what a valid ID is (between 1 and 255) by checking it - having
a separate type allows us to centralize that check for validity.

IDs need to be allocated at an RTP transport level. Manufacturing your
own ID is a footgun - sooner or later the ID you manufacture will be
used by the allocator, leading to an unexpected error situation. Having
a separate type allows the reader to easily figure out where IDs are
being created, and verify that it's either in an unit test or in the
allocator.

Key changes:
- RtpHeaderExtensionId defined in api/rtp_header_extension_id.h
- Backwards compatible overloads (taking int) added to public APIs in
  api/ and modules/rtp_rtcp/ to ensure no breaking changes for
  downstream projects.
- RtpPacket, RtpHeaderExtensionPicker, and other internal components
  updated to use the new type.
- Updated SRTP components to handle RtpHeaderExtensionId.
- Fixed naming conflict in fake_rtp.h by renaming kRtcpReport to
  kFakeRtcpReport.
- Updated numerous tests to comply with the new explicit type.

Bug: webrtc:514817938
Change-Id: I79eb728f2ebd5c1fe5a1711eae2162c742fd029c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/473880
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47832}