Add support/handling for FirebaseAuthWeakPasswordException

[Bradleycorn]

Addresses #289 by adding support for the FirebaseAuthWeakPasswordException (https://firebase.google.com/docs/reference/kotlin/com/google/firebase/auth/FirebaseAuthWeakPasswordException) in the firebase-auth module.

• Marks FirebaseAuthInvalidCredentialsException as open so that it can be inherited from (more info below).
• Adds a new class FirebaseAuthWeakPasswordException that inherits from FirebaseAuthInvalidCredentialsException. This is done for two reasons:
  1. In Firebase itself, the weak password exception inherits from the invalid credentials exception. This keeps that same structure in this library.
  2. It prevents breaking changes. Existing code in consumer applications that is expecting FirebaseAuthInvalidCredentialsException will continue to function normally, since the new FirebaseAuthWeakPasswordException IS a FirebaseAuthInvalidCredentialsException.
• Implements the FirebaseAuthWeakPasswordException for all platforms (android, iOS, js).

[Bradleycorn]

@Reedyuk - The assemble task on the Pull Request build has been running for 5 hours .. That seems ... not right. Any ideas? I can ./gradlew assemble this branch on my machine and it builds fine.

[Reedyuk]

I've cancelled it and reran it, there is a pr with kotlin bump to fix it

[nbransby]

good stuff, thanks @Bradleycorn