Add / Assign custom domains to functions programatically

### What problem are you trying to solve?

I want to build a CDN on top of deno deploy. This requires my customers to have it running on their domain so that they can send the cookies along safely. I would need to add every single domain by hand which doesn't scale!

### Describe the solution you'd like

Some simple API which allows me to add and assign a custom domain to my deno deploy function.

### Describe alternatives you've considered

I could potentially just reverse engineer this how it's currently done but it's probably error-prone since the API could always change. Also tokens might be an issue

### Documentation, Adoption, Migration Strategy

_No response_