FR: Add Custom Domain Support

### Feature proposal

* Firebase Component: RemoteConfig

On the current project I am working on, we require certain traffic to be zero-rated. One of those is RemoteConfig. A cheeky way of zero-rating the traffic is to set up a custom domain that redirects traffic to Firebase RemoteConfig. However, to do this, we would require the SDK to support using a custom domain as one of its setttings.

Thoughts?

No files, tests, or entry points are named. Start by locating the Firebase RemoteConfig SDK configuration and how its service endpoint is selected; then determine the supported custom-domain behavior and its constraints. Done should be defined by an agreed design and tests covering custom-domain configuration, but the issue does not specify either.

Written by the indexing model from the issue text.