Format of response to POST /_matrix/identity/v2/store-invite is unclear

[richvdh]

Link to problem area:

https://spec.matrix.org/v1.5/identity-service-api/#post_matrixidentityv2store-invite

Issue

The public_keys response field is documented as having type [string] and description "A list of [server’s long-term public key, generated ephemeral public key]."

The type is flat-out wrong, and the description is insufficient. In practice, Sydent returns:

{
    "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"
        }
    ]
}

[richvdh]

Duplicate of #495