Cache ICE credentials and remove signaling thread blocking calls

Avoid thread hops to the network thread by caching pooled ICE
credentials on the signaling thread. Previously, generating SDP offers
or answers required synchronous blocking calls to retrieve credentials
from the PortAllocator.

Key changes:

* Update JsepTransportController::MaybeStartGathering to return
  pooled credentials from the network thread.
* Extend PeerConnection and SdpOfferAnswerHandler to store and
  propagate these credentials to session options without blocking.
* Update tests and fakes to support the new credential retrieval flow
  and proper PortAllocator lifecycle management.

Bug: webrtc:42222117
Change-Id: Ib66bb074ae9b7ee33b6033ce31c3f0746897a4f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/463560
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47443}