WIP: task profiling

[vaind]

This changes the profiler to track tasks across threadpools. It makes use of ActivityComputer, which is currently only patched in our fork and not upstream.

Also, there's currently a blocking issue because with a recent change in TraceLog to clean data structures after a sample is processed (see #3375 (comment)). The problem is that the ActivityComputer relies on reading older stacks when figuring out creator stacktraces (AFAICT). We'd need to find solution for that to make the ActivityComputer work with realtime sessions, while not reverting the indefinitely growing data structures.
Maybe, we could make ActivityComputer process and keep the needed stack trace information at the time that sample occurs, if that is possible?
In any case, I'd wait until the perfview PR with the main changes we need is merged, before opening another set of changes.

[github-actions]

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