feat: add support for custom external registry

[ewgenius]

Allows to pull components from custom registry url:

npx shadcn-ui@latest add my-custom-component --registry https://ui.example.com

[vercel[bot]]

@ewgenius is attempting to deploy a commit to the shadcn-pro Team on Vercel.

A member of the Team first needs to authorize it.

[shadcn]

Keeping an eye on the work here. If we're doing this, we need to come up with a registry schema.