Stable MSC4284 implementation

Call /sign instead of /check as a fallback (and persist signatures)
Ensure events (soft) fail rather than pass due to lack of signatures
For 3-6 months, fall back to unstable /sign
- Remove unstable MSC4284 support #19502
Proxy error responses from policy server to client-server API
Support stable event type m.room.policy
Fix shallow update when signatures are on the same domain

@turt2live

@turt2live since #19503 will close this issue, can you create another one to track the "Ensure membership events are checked properly" task please?

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