Firebase auth Email Action Links API

[hardikns]

Added generate_password_reset_link(),
generate_email_verification_link() and generate_email_sign_in_link()
methods to the auth API.

[hiranya911]

@hardikns, thanks for putting this together. We already have a reviewed and approved API proposal for this. Can you change you implementation to fit the following API signatures:

generate_password_reset_link(email, action_code_settings=None, app=None)
generate_email_verification_link(email, action_code_settings=None, app=None)
generate_sign_in_with_email_link(email, action_code_settings, app=None)

ActionCodeSettings(url, handle_code_in_app=None, ios_bundle_id=None, android_package_name=None, android_install_app=None, android_minimum_version=None, dynamic_link_domain=None)

I left a few other comments after taking a quick peek at the code. Once you've made the necessary changes to the API, I will take do a more thorough review.

[hiranya911]

Thanks @hardikns. The API looks correct. I've left a bunch of comments on the implementation -- mostly minor refactorings.

[hardikns]

@hiranya911 , Thanks. I will address these comments in couple of days.

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[googlebot]

CLAs look good, thanks!

[hardikns]

@hiranya911 , I think I have fixed all comments, but will re-review tomorrow.

[hiranya911]

Thanks @hardikns. Looks pretty solid. Just a couple of nits in the integration tests and then I can merge this.

[hiranya911]

@hardikns I'm approving this PR with a couple of suggestions for cosmetic improvements. I will merge the PR when those last bits are addressed. Thanks again for all your work on this PR.