Expose defer_to_threadpool in the module API

[anoadragon453]

Exposes defer_to_threadpool in the Module API, so Synapse modules can run tasks on a thread without colliding with tasks that Synapse itself is trying to schedule.

The motivation for this change is to allow synapse-s3-storage-provider to continue to use its own threadpool while switching its implementation away from make_deferred_yieldable.

Supersedes #19024.

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