Implement room v12 (#943)

**Does not yet work!** Currently, state resolution does not correctly resolve conflicting states. Everything else appears to work as expected, so stateres will be fixed soon, then we should be clear for takeoff.

Also: a lot of things currently accept a nullable room ID that really just don't need to. This will need tidying up before merge. Some authentication checks have also been disabled temporarily but nothing important.

A lot of things are tagged with `TODO(hydra)`, those need resolving before merge. External contributors should PR to the `hydra/public` branch, *not* ` main`.

---

This PR should be squash merged.

Reviewed-on: https://forgejo.ellis.link/continuwuation/continuwuity/pulls/943
Co-authored-by: nexy7574 <git@nexy7574.co.uk>
Co-committed-by: nexy7574 <git@nexy7574.co.uk>

src/api/client/sync/v3.rs

@@ -457,7 +457,7 @@ async fn handle_left_room(
 			state_key: Some(sender_user.as_str().into()),
 			unsigned: None,
 			// The following keys are dropped on conversion
-			room_id: room_id.clone(),
+			room_id: Some(room_id.clone()),
 			prev_events: vec![],
 			depth: uint!(1),
 			auth_events: vec![],