Need to implement method to verify that only a single additional invite can be included when calling createRoom

[jason-famedly]

Per the third paragraph here: https://github.com/gematik/api-ti-messenger/blob/main/docs/Fachdienst/MessengerService.adoc#21-client-server-pr%C3%BCfungen

The following checks must be carried out by the messenger proxy :

• The messenger proxy must ensure that the attribute is filled invite with a maximum of one MXID when a room is created.
• For each request, the messenger proxy Invite-Event must check whether the matrix domains contained in the request belong to the TI federation.
• For incoming communication to the media endpoints, the messenger proxy must check the path component {serverName} for federation membership

(Translation by Google, formatting is mine)

The first of these should be implemented in this code base. Out-of-band discussion with @nico-famedly has led to the conclusion that a third party rules callback for on_create_room() would be an appropriate method to handle this.