MSC4242: State DAGs (Federation)

[kegsay]

Builds off #19424

Adds federation compatibility for MSC4242 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.

Complement tests: matrix-org/complement#841

Pull Request Checklist

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