Dynamic discovery url handling

[testower]

I'm planning to remove the current "gbfs-internal" endpoint feature and replace it with dynamic url handling based on the Host header.

The reason we have this "gbfs-internal" endpoint is that we have a separate caching service between our instance of lamassu and our OTP instance. The hostname of this internal service is configured in lamassu so that the caching service can point to the gbfs-internal endpoint and receive discovery files with URLs rewritten to use that hostname.

I think we can instead bake this feature into the main endpoint by reading the Host header and rewriting urls based on that.