Enable full phone MFA flow for emulator requests

[dantheli]

Discussion

Fixes #11079 by letting the phone auth provider create a MFA sign in request even if connected to the emulator.

This puts the "send code" logic into a helper method on FIRPhoneAuthProvider that accepts appCredential and reCAPTCHAToken and is called in both emulator and production paths. In the emulator path, these are nil, as the emulator expects.

Testing

Auth tests pass, but I have not written a test for this flow yet.

API Changes

None

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[paulb777]

Thanks for the fix!

Tests would be great. If you'd prefer to write in Swift, feel free to send a PR to the auth-swift branch which will replace the Auth ObjC implementation with a Swift one.

[dantheli]

Oh wow how exciting! When is that planned to be merged? Objc & OCMock have until very recently been a large part of my life for some time so I don't mind, but if the Swift implementation will be merged soon, I'll redo this over on that branch.

[paulb777]

We're hoping to do in the next few months - but still some open questions about timing.

In the meantime, we should be able to go forward with the ObjC version

[paulb777]

And would appreciate another PR on the auth-swift branch!