FR: Validate phone number in the server

I'd like to suggest a way to validade the phone number in the server.
Something like `validatePhoneNumber(verificationId, verificationCode)` (which returns the phone number, for example) would be very useful.

Maybe also a `validatePhoneNumber(verificationId, verificationCode, phoneNumber)` that returns a Boolean.

I don't see how to verify if the phone is valid without creating an account. The client has sent the sms, the user has submited the verification code, but the only methods available are to `link` or `signin` (which would create an account with `sJks92mda0` uid).

> PS: A way to set/change the uid of a firebase user would also be a workaround to me, because then I would be able to create the account in the client and change the uid in the server.

* Operating System version: macOS Sierra 10.12.6
* Firebase-admin version: 5.0.1
* Firebase Product: auth