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

### 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:
```swift
Auth.auth().validatePassword('password123')
```

### Firebase Product(s)

Authentication

Start with the password-policy documentation and the referenced firebase-js-sdk packages/auth/src/core/index.ts implementation. Then inspect the iOS SDK's existing Auth API entry points to determine how client-side validation would fit. Done means agreeing on an iOS API and providing password-policy feedback during email/password signup.

Written by the indexing model from the issue text.