Refactor custom domain API in the `deploy` package

[infomiho]

Based on https://github.com/wasp-lang/wasp/pull/2578#discussion_r2077450022

We should probably revisit the way we handle custom domains when users deploy their apps with Wasp CLI.

Users can define a REACT_APP_API_URL env variable that the Wasp CLI and the deploy package detect and use instead of the default {prefix}-client domain. That API feels little bit too low level in hindsight. It was a hotfix proposed by a community member who was blocked and we accepted it.

We should think about the API and see if we can come up with something better.