Firebase Auth/Import Users: added a phone number to the user provider

[mymyparty]

This MR adds a phoneNumber field to the user provider struct.
Related issue: #540
There is no relevant existing test.

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[lahirumaramba]

Hi @mymyparty Thank you for your contribution. Please change the base branch to dev before we can start the review process.

https://github.com/firebase/firebase-admin-go/blob/master/CONTRIBUTING.md#want-to-submit-a-pull-request

Make sure to create all your pull requests against the dev branch. All development work takes place on this branch, while the master branch is dedicated for released stable code. This enables us to review and merge routine code changes, without impacting downstream applications that are building against our master branch.

[mymyparty]

@lahirumaramba done

[mymyparty]

@lahirumaramba Hi! Any progress on this?