Spec per AS publicRooms list

[erikjohnston]

(Depends on #868)

AS's can publish rooms into their own public room list, so that they can advertise public rooms for their network without filling up the main public room list.

There are two sides to this:

1. the new client APIs for querying in /publicRooms
2. the new AS APIs that allow AS's to publish and unpublish rooms.

See matrix-org/synapse#1676 for details