[firebase_auth] Add updatePhoneNumber function

[zerefati]

Add updatePhoneNumber function.

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[zerefati]

I signed it!

[googlebot]

CLAs look good, thanks!

[kroikie]

@zerefati Thanks for the contribution, we would need to have the Android and iOS implementation of this before we can merge, any chance you can add that?

[collinjackson]

@zerefati - friendly ping, please let us know if you want to finish or abandon this PR

[zerefati]

Yes sure, I will update it soon.

[GianlucaCesari]

Is this going to be in the next package release?

[collinjackson]

I am hoping to land and publish it soon, but it needs a unit test still. If anyone wants to write that one, it will help me merge the PR faster.

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[GianlucaCesari]

I'm ok with my commits being contributed to this project.

[collinjackson]

This change LGTM, just waiting for the tests to pass.

[GianlucaCesari]

Hey guys, what's missing to approve this PR?