Add experimental support for MSC4308: Thread Subscriptions extension to Sliding Sync when MSC4306 and MSC4186 are enabled.

[reivilibre]

Closes: #18436

Implements: matrix-org/matrix-spec-proposals#4308

Follows: #18674

This pull request is intended for commit-by-commit review.

1. Update simplified sliding sync docstring

2. spelling

3. Add models for Thread Subscriptions extension to Sliding Sync

4. Add overload for gather_optional_coroutines/6

5. Add thread subscriptions position to StreamToken

6. Add subscribed and automatic to get_updated_thread_subscriptions_for_user

7. Implement MSC4308

8. Put MSC4308 behind the MSC4306 feature flag

[MadLittleMods]

(I have looked things over but haven't given everything a proper review-level scrutiny)

[MadLittleMods]

I'd rather not do this sort of thing. This is the sort of pattern that made legacy /sync complex, brittle, and unpredictable.

What's spurring on this change?

Paginating incremental sync is something unsolved in the Sliding Sync MSC and I don't think we should solve it here.

[reivilibre]

The intention of this is we don't want to have to fetch and send an unbounded number of thread subscriptions down to the client.

How else would you suggest doing this?

[MadLittleMods]

One option is to have the limit work how timeline_limit works. We pass back messages up to the timeline_limit and a prev_batch token to paginate back from there. Hence my comment about an API endpoint to paginate thread subscriptions -> matrix-org/matrix-spec-proposals#4306 (comment)