Publish publicRooms endpoint

[tcpipuk]

A number of people with vanity domains want a useful way to publish these room aliases as a public room directory so they can be indexed by projects such as matrixrooms.info.

Currently it's possible to get this functionality using a static file alongside mauliasproxy, but undoubtedly it would be helpful if this project also included that endpoint to have the alias and public directories configured in one place.

[tcpipuk]

(apologies my Go skills are nonexistent, or I'd have an attempt at writing this for a PR)