Add room subscriptions to Sliding Sync /sync (#17432)

Add room subscriptions to Sliding Sync `/sync`

Based on
[MSC3575](matrix-org/matrix-spec-proposals#3575):
Sliding Sync

Currently, you can only subscribe to rooms you have had *any* membership
in before.

In the future, we will allow `world_readable` rooms to be subscribed to
without joining.

Author: MadLittleMods

changelog.d/17432.feature

@@ -0,0 +1 @@
+Add room subscriptions to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.