Split RTPSender packet building into helpers

The code that attaches the MID and RID header extensions was duplicated
in AllocatePacket(), BuildRtxPacket() and both padding paths of
GeneratePadding(), and GeneratePadding() had grown long and hard to
read.

- Move the MID and RID logic into AttachMidAndRid().
- Build media and padding packets in BuildMediaPacket() and
  BuildPaddingPacket(), and track the maximum CSRC count in
  UpdateMaxNumCsrcs().
- Split GeneratePadding() with AppendPayloadPadding() and
  PaddingBytesInPacket(). Checks that cannot change inside the padding
  loop now return before the loop instead of breaking out of it.

The helpers still run under send_mutex_. This prepares
https://webrtc-review.googlesource.com/c/src/+/507280, which builds
packets after releasing the lock and then only needs to change which
state the helpers read. BuildMediaPacket() and UpdateMaxNumCsrcs() will
also be used by a per-frame variant of AllocatePacket().

No change in behavior.

Bug: webrtc:42223727
Change-Id: I22b7682a65fb7c56d1c92b8cd07af3ef998b0b44
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507642
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48809}