[firebase_firestore] useEmulator request

[radvansky-tomas]

I have issue using firebase firestore with hot reload and thats because we have to use .settings and not useEmulator as with other packages such as auth / functions.

Could we add / change this behaviour so it follows official SDKs:

https://firebase.google.com/docs/emulator-suite/connect_firestore#android_ios_and_web_sdks

// Firebase previously initialized using firebase.initializeApp().
var db = firebase.firestore();
if (location.hostname === "localhost") {
  db.useEmulator("localhost", 8080);
}

[Salakar]

cc @rrousselGit @russellwheatley - this would be a breaking change (though we can keep Settings with a deprecation decorator) but also is something I think we should do to match the other FlutterFire APIs from Auth & Functions (useEmulator) - changing it would also match the latest Firebase Web/Android/iOS naming too.

Here's the docs on how it does it currently; https://firebase.flutter.dev/docs/firestore/usage#emulator-usage vs Auth; https://firebase.flutter.dev/docs/auth/usage#emulator-usage vs Functions; https://firebase.flutter.dev/docs/functions/usage#emulator-usage

We should also probably deprecate the Functions one (useFunctionsEmulator) as that should also be changed to useEmulator. The APIs across all three are different (different name or args and also named vs positional etc) - I think we should standardise the names and the arguments.

[Salakar]

Tracking on #5224