Is your feature request related to a problem? Please describe:
I use two-factor authentication in my app and would love it if you could implement verifyPhoneNumber. I'm not sure if signInWithPhoneNumber would work in this case (I haven't tried it yet);

Describe the solution you'd like:

To be able to generate a verificationId with verifyPhoneNumber.

Describe alternatives you've considered:

I've tried using the firebase sdk and it works on web and Android but iOS won't load the recaptcha needed to verify the phone number and generate the verificationId.