Use asynchronous posting for PeerConnection signaling tasks

Update RunOnSignalingThread to use PostTask with thread safety flags
instead of BlockingCall. The places where RunOnSignalingThread is used,
bypass the proxy but used BlockingCall to remain functionally compatible
with the proxies. The methods are however asynchronous, so the
BlockingCall can be avoided. That was not done in the first step when
RunOnSignalingThread was introduced in order to not bring too many
changes (the main focus was on allowing non thread-hopping getters).
This change is a follow up step that transitions the initial step of the
task execution from a synchronous, blocking operation to an asynchronous
when called from a different thread.

As a btw change, SetLocalDescription and SetRemoteDescription now
consistently manage the lifetime of their arguments using
std::unique_ptr and scoped_refptr to ensure that
SessionDescriptionInterface and SetSessionDescriptionObserver objects
remain valid until the asynchronous task completes on the signaling
thread, preventing potential use-after-free or memory leak issues.

Bug: webrtc:442220720
Change-Id: I71cda2643c8ddd89622436df08bf889c0a35c56d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454980
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47095}