Builds off https://github.com/element-hq/synapse/pull/19424
Adds federation compatibility for state DAG rooms. Overview:
- Adds extra HTTP API fields as per the MSC.
- Adds methods for walking and extracting the state DAG for a room (for `/get_missing_events` and `/send_join` respectively).
- Adds impl for processing the federation requests, as well as `/send`.