Allow modules to provide local `/publicRooms` results

This issue has been migrated from [#15518](https://github.com/matrix-org/synapse/pull/15518).

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This PR creates a new module callback, `fetch_public_rooms`, which allows modules to inject results into the responses of `GET/POST /_matrix/client/v3/publicRooms` [on the Client-Server API](https://spec.matrix.org/v1.6/client-server-api/) and the [Federation API](https://spec.matrix.org/v1.6/server-server-api/#post_matrixfederationv1publicrooms).

This work supports the ongoing work for DINUM to write a Synapse module to aggregate `/publicRooms` information from multiple homeservers for a client. Internal JIRA link for those with access: https://element-io.atlassian.net/browse/PSP-2109.

Read the Client-Server and Federation API sections linked in the issue, then trace the existing GET/POST /_matrix/client/v3/publicRooms and federation public-rooms handlers. Done means modules can implement fetch_public_rooms and its results appear in both response paths.

Written by the indexing model from the issue text.