Update passwordHash and passwordSalt to be base64 decoded before user import

[NhienLam]

Issue: Using the importWithScrypt() code snippet in the public doc will successfully import the user, but we will be unable to sign in because the password is changed.

This may be similar to this issue in Firebase Admin Go: https://github.com/firebase/firebase-admin-go/issues/479 and the fixes in https://github.com/firebase/firebase-admin-go/pull/480 and https://github.com/firebase/firebase-admin-python/pull/652

After this change, we can successfully import a user and sign in with the original password.

Note: I use BaseEncoding.base64() instead of BaseEncoding.base64Url() because BaseEncoding.base64Url().decode will throw an Unrecognized character: + exception if the password hash contains '+'.

[lahirumaramba]

Thank you @NhienLam ! LGTM!

Do you think we should update other import snippets, too? importWithBcrypt etc.

[NhienLam]

Do you think we should update other import snippets, too? importWithBcrypt etc.

Thanks @lahirumaramba! I think the other import snippets don't need to be changed because they are using other hashing algorithms. Quote from firebase/firebase-admin-python#652: "It is fair to assume that the password hash and salt from those other backends has been decoded properly."