iroh-relay: support the relay server on a URL prefix

[flub]

Currently we only support a RelayUrl with a toplevel domain name: https://example.com/. Internally this will results in several routes being served, like https://example.com/relay etc.

Perhaps we should consider allowing it to be hosted on any custom path prefix as well: https://example.com/path/prefix/ so that the routes served are "mounted" at this location: https://example.com/path/prefix/relay etc.

This could make sense in some deployments where the same domain wants to host other resources.