Refactor state caching for SctpDataChannel observers

Replace the internal CachedGetters logic in ObserverAdapter with a
simplified CachedState mechanism in SctpDataChannel.

CacheStateAndCallBackOnSignalingThread captures the channel's state and
error on the network thread before posting a task to the signaling
thread. These values are temporarily cached and returned by the state()
and error() getters during the callback's execution.

This ensures that observers receive the correct state values captured at
the time of the event and avoids thread hops to read the state/error and
also removes some boilerplate ObserverAdapter implementation.

Bug: webrtc:510487699
Change-Id: I62a303efa26c8e84a56feb678d715aac5c8ac433
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/369142
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47859}