add new site-id and custom-url options

This will add two new options, `site-id` and `custom-url`. Used in tandem, you can have RF runs over some ad-hoc staging or QA environments.

This is a WIP and just adds the options and doc, it does nothing yet.
It was not handling errors at all, now it is. There will be a new PR on main app to add some nicer URL errors too.