Add Sliding Sync `/sync/e2ee` endpoint for To-Device messages (#17167)

This is being introduced as part of Sliding Sync but doesn't have any sliding window component. It's just a way to get E2EE events without having to sit through a big initial sync  (`/sync` v2). And we can avoid encryption events being backed up by the main sync response or vice-versa.

Part of some Sliding Sync simplification/experimentation. See [this discussion](https://github.com/element-hq/synapse/pull/17167#discussion_r1610495866) for why it may not be as useful as we thought.

Based on:

 - https://github.com/matrix-org/matrix-spec-proposals/pull/3575
 - https://github.com/matrix-org/matrix-spec-proposals/pull/3885
 - https://github.com/matrix-org/matrix-spec-proposals/pull/3884