[firebase_auth] Confirm password reset

[lanmonster]

Description

This PR gives the ability to confirm password reset in order to control the entire "forgot password" flow from within an app. All of the pieces were there for the platforms, so this PR is just hooking up the plumbing to be able to access from dart.

Related Issues

#1739

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[lanmonster]

closing to favor splitting into two separate PRs. one for the platform_interface side, and then when that lands, one for the firebase_auth side.