Fix possible uaf in WebRtcSessionDescriptionFactory::Post

Move the callback out of the callbacks_ queue before invoking it. This
prevents access to the destroyed callbacks_ queue if the callback
synchronously destroys the factory.

Also convert the on_certificate_ready callback from std::function to
absl::AnyInvocable (move-only) and add a validation check in
constructor.

Bug: b/428836165, b/515407473
Change-Id: I433e0f908dd7737924db1bfe0839b2be699f7517
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/478842
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47935}