Add CLI feature for adding a custom domain

[jph00]

It would be helpful to have something in the CLI to allow us to add a custom domain. AFAICT current railway domain only adds an internal one. I'd like to be able to automate the process of attaching a custom domain.

[JakeCooper]

I'm happy to throw a community bounty up on this one for $100. Payable via GH sponsors/BMAC/etc