[FR]: Add support for password validation against password policy

[rebeccachin]

Description

1. During email/password signup, user can receive feedback if the password doesn't fulfill requirements
2. I can't find an API for this in the iOS SDK
3. I see that the firebase-js-sdk supports this feature.
   • https://cloud.google.com/identity-platform/docs/password-policy#enforcing_on_the_client_side)
   • https://github.com/firebase/firebase-js-sdk/blob/62348e116c795d19c5ca58729c250805240ce345/packages/auth/src/core/index.ts#L124-L130

API Proposal

usage might look like:

Auth.auth().validatePassword('password123')

Firebase Product(s)

Authentication

[google-oss-bot]

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