Spec unclear on how to make a `key_validity_url` for `m.room.third_party_invite` at the root level

`/store-invite` doesn't return it (but does return `public_key` at the root: ~~https://github.com/matrix-org/matrix-doc/issues/2947#issuecomment-756240264~~ https://github.com/matrix-org/sydent/issues/561) - should the client/server assume it's always `https://identity.example.org/_matrix/identity/v2/pubkey/isvalid`? (this is what Synapse appears to do)

Start by reading the Matrix specification section for m.room.third_party_invite and the /store-invite behavior described in the issue. Compare the linked matrix-doc and Sydent discussions with Synapse's apparent default URL. Done means the specification clearly defines how clients and servers obtain or construct key_validity_url.

Written by the indexing model from the issue text.