Small step towards using separate classes for TCP server sockets.
Added a new test-only class AsyncStunServerTCPSocket needed
for unit tests of AsyncStunTCPSocket.
Bug: webrtc:13065
Change-Id: I7d9713983d8f6b30aa3d3e7442bb34ea48b815eb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/232324
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35047}