Refactor ModuleRtpRtcpImpl2 to use static factory methods

This change makes the ModuleRtpRtcpImpl2 constructor private and introduces
CreateSendModule and CreateReceiveModule static factory methods.
This enforces the separation of sender and receiver modules at the API level
and ensures that the 'receiver_only' flag in the configuration matches the
intended usage.

All call sites and members have been updated to use these factory methods
and hold the module via std::unique_ptr.

Bug: webrtc:41480926
Change-Id: I60f5a8417bab9fafdb8ca2c8396d1421838e3a8a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/443200
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46717}