[🐛] Functions SDK Inconsistency

[mbleigh]

Issue

The iOS, Android, and JS SDKs for Firebase now support custom domain endpoints. In JS it looks like this:

const someFunc = firebase.functions("https://my.custom.url").httpsCallable("someFunc");
await someFunc({data: 'here'}); // calls https://my.custom.url/someFunc instead of cloudfunctions.net URL

This was a fairly recent addition since previously only a region could be passed to the factory method. It would be nice for the React Native library to catch up to the change in the other SDKs. Thanks!

[Salakar]

Thanks @mbleigh we'll get this looked at.

@dackers86 or @russellwheatley - would you mind taking a look at this this week, I've added an internal ticket on the new board to track (RNFB-1).