fetch_public_rooms is a module API callback intended to be used when a
request for the homeserver's public rooms list comes in via either the
CS or SS API. Modules can return an ordered array of public rooms that
they would like to inject into the list supplied by the homeserver.
This can be useful for exposing known rooms that users on the
homeserver have not joined yet, and the property of mixing with the
normal public rooms list is desirable versus the solution of creating
a new third-party network type to load your rooms under.
Mathieu Velten