Support registering guests against a different homeserver to the default homeserver in the config.

### Your use case

#### What would you like to do?

The ability to specify registering guests users against a different homeserver to the default homeserver. I.e. a `guests.half-shot.uk` for guests, but still allowing regular registrations against `half-shot.uk`.

#### Why would you like to do it?

Some large conferences like to have an attendees server and a main server. They would like their guests to be registered against the main server, but their conference-goers to be against the attendee server.

#### How would you like to achieve it?

Config option to state the default guest homeserver.

### Have you considered any alternatives?

Obviously, you could just have the guests on the attendee homeserver. That does unfortunately not meet the requirement.

### Additional context

_No response_