Reland "Remove blocking calls from the stats gathering path in RTCStatsCollector"

This is a reland of commit c2ffdeee34b23dc8a69360e8ed80d2b276d72377

The main fix related to the revert/reland was landed in a previous
reland of a CL that this CL was based on. An additional change is in
patchset #2 which preserves the previous behavior (not issuing callbacks
during shutdown although it theoretically might be possible).

Original change's description:
> Remove blocking calls from the stats gathering path in RTCStatsCollector
>
> Improve tests for CancelPendingRequestAndGetShutdownTasks to more
> accurately simulate multithreaded shutdown. Any pending operations are
> cancelled as before, but CancelPendingRequestAndGetShutdownTasks will
> now optionally issue the pending callbacks immediately while also
> canceling pending async operations without blocking.
>
> Remove WaitForPendingRequest which was previously called from PC::Close.
> The dependency on the transport controller is not a concern anymore.
>
> * Introduce `worker_safety_` to guard tasks running on the worker
>   thread.
> * Refactor `PrepareTransceiverStatsInfosAndCallStats_s_w` to return
>   an `absl::AnyInvocable` and pass data via function arguments
>   instead of storing intermediate results in `collection_context_`.
> * Remove `network_report_event_` as manual synchronization is no
>   longer needed.
> * Ensure pending requests receive callbacks immediately during
>   shutdown or are safely dropped if the collector is destroyed.
>
> Bug: webrtc:42222804
> Change-Id: Ibb576e46dfa77e95c072d99282f6780a6df46458
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/443460
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46747}

Bug: webrtc:42222804
Change-Id: I65664d832145a346e445fc6246a6c1ec54592d96
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444440
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46784}