Sliding Sync: Include invite, ban, kick, targets when $LAZY-loading room members

[MadLittleMods]

Sliding Sync: Sliding Sync: Include invite, ban, kick, targets when $LAZY-loading room members.

Part of #17929

Previous description

This ensures that we send down membership updates whenever we see it change in the timeline. This allows clients to cache the membership list for as long as it doesn't get a gappy sync, but still ensures for large gaps the server doesn't need to send down all membership changes.

Pull Request Checklist

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[erikjohnston]

This makes sense to ensure that the client always gets these relevant membership events.

However, this will still filter out membership changes that don't appear in the timeline. For that case I think we need to not filter membership changes at all for non-gappy incremental syncs (and then make sure the _required_state_changes behaves appropriately).

[MadLittleMods]

@erikjohnston Can you provide an example?

if a user's state changes and it is not in the timeline

@erikjohnston When does this happen? If someone changes their display name for example, the membership state event will show up in the timeline. And if the membership change happened outside of the range of the timeline_limit, then we are talking about a limited: true sync where this new logic doesn't apply anyway.

Is this targeting a scenario where some state comes over federation from some time ago which updates the current state but isn't in the timeline? (not sure this is even a thing because it will still get a recent stream_ordering which should make it come down /sync)

-- #17929 (comment)

[erikjohnston]

This can happen due to state res, where a membership e.g. gets reverted to a previous state. In that case we won't have an entry in the timeline, but will have a state event in the state deltas

[MadLittleMods]

Ahh, so in state reset scenarios. I think we will need to merge #17732 as a prerequisite to handle those cases (or maybe not but it's related).

[MadLittleMods]

Thanks for the review @erikjohnston 🦋