Update sliding sync to latest

[gnunicorn]

This adds support for the latest spec proposal updates of MSC 3575 ("Sliding Sync") for:

• tags and not_tags filters
• resolve_tombstones flag
• limited, joined_count and invited_count responses taken over from v3

This also adds the current state of existing extensions. These are not yet specified, even within the proposal but are not likely to change much going forward. Those are:

• ToDevice Messages
• End-to-end-encryption messages
• AccountData, globally as well as for specific rooms.

This is all still behind the same feature-flag as before.

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Preview Removed

[jplatte]

Ah, I thought you adressed all review comments and we could merge with that one extra change. Looks like one last change (pub use) and re-format is still needed.

[gnunicorn]

No, I had to do fixes for the currently buggy proxy-implementation. Not entirely sure if we want to merge that as is... your call.

[jplatte]

It's an unstable feature, so I'm happy to merge after the breaking change to the stable API is fixed (DeviceLists not being exported from sync_events::v3).

[gnunicorn]

weird, I made the change, but looks like I forgot to merge.

however, meanwhile the proxy bug is supposedly fixed. So let me confirm that and remove the hacks...