sign3pidInvitation

abstract suspend fun sign3pidInvitation(    identiyServer: String,     token: String,     secret: String): SignInvitationResult

When one performs a 3pid invite and the third party identifier is unknown, the home server will store the invitation in the Identity server and store some information in the room state membership event. The email invite will contains the token and secret that can be used to claim the stored invitation.

To aid clients who may not be able to perform crypto themselves, the identity server offers some crypto functionality to help in accepting invitations. This is less secure than the client doing it itself, but may be useful where this isn't possible.