Enhance ScopedOperationsBatcher to support tasks returning tasks.

A common pattern for negotiation operations that involve the signaling
thread as well as one or both of the worker/network threads, is to a)
apply partial state on the signaling thread, b) perform work on the
worker/network thread(s), c) run completion work on the signaling thread
that depends on the outcome of the previous step.

This adds support for that pattern to the ScopedOperationsBatcher so
that this pattern can be applied to batched operations.

Bug: webrtc:42222804
Change-Id: I776786216208991c1716a1ab2808c9b37f30ead8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449362
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47195}