Add a signing-key pinning mechanism to the federation key APIs (SPEC-329)

Add a way for home-server operators to promise in the /key responses that they won't lose the private keys for their HS.

Then other HSes that have observed that key won't accept a different key for that domain, giving stronger guarantees against MITM attacks.

Something like https://en.wikipedia.org/wiki/HTTP_Public_Key_Pinning

(Imported from https://matrix.org/jira/browse/SPEC-329)

(Reported by @NegativeMjark)

Start with the federation /key responses described in the issue and review the linked HTTP Public Key Pinning background. Define the signing-key pinning mechanism, including how home servers promise key retention and how other servers handle a changed key. Done means the federation key API specification clearly documents the behavior and its security guarantees.

Written by the indexing model from the issue text.