At the moment we use the convention that all RPCs between federator -> brig/galley/etc use the "federation" path prefix. This PR adds validation to ensure other paths cannot be called, as that would expose non-federation APIs (including internal, non-authenticated ones) to the world.

See also the discussion in https://wearezeta.atlassian.net/wiki/spaces/CORE/pages/224166764/Limiting+access+to+federation+endpoints