POST /_matrix/identity/v2/store-invite returns v1 key validity APIs

[richvdh]

Sydent's handler for POST /_matrix/identity/v2/store-invite returns:

{
    "token": "token",
    "public_key": "sydentPubKeyBase64",
    "public_keys": [
        {
            "public_key": "sydentPubKeyBase64",
            "key_validity_url":  "https://<server>/_matrix/identity/api/v1/pubkey/isvalid"
        },
        {
            "public_key": "ephemeralPublicKeyBase64",
            "key_validity_url": "https://<server>/_matrix/identity/api/v1/pubkey/ephemeral/isvalid",
        }
    ],
    "display_name": "<redacted email>"
}

As this shows, the key_validity_urls are under the now-defunct /api/v1 path. Arguably, this doesn't matter, because:

• When populating the m.room.third_party_invite event, Synapse also builds its own key_validity_url and stores it at the top level of the event, where it is given precedence over the key_validity_urls within public_keys.
• This is a Sydent URL generated by Sydent, so it doesn't need to be part of the spec. However, if we want to make that argument, it should be outside the /_matrix/ namespace anyway (cf /_matrix/identity/api/v1/sign-ed25519 links in emails should use non-/_matrix namespace and have a better name #533).

[richvdh]

Related: matrix-org/synapse#6036, matrix-org/matrix-spec#1361.

[richvdh]

Related: matrix-org/matrix-spec#495, #338