Implement firebase-js-sdk on `Auth.config` object to expose the current authDomain related data elements

[mikehardy]

The only thing holding this back is I realized there is an API shape for firebase-js-sdk on Auth.config where I could (and should) expose the current authDomain but it has several other data elements that are valid and separate and I'm not sure how to populate them.

Originally posted by @mikehardy in #7530 (comment)

[github-actions]

Hello 👋, to help manage issues we automatically close stale issues.

This issue has been automatically marked as stale because it has not had activity for quite some time.Has this issue been fixed, or does it still require attention?

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[github-actions]

Hello 👋, to help manage issues we automatically close stale issues.

This issue has been automatically marked as stale because it has not had activity for quite some time.Has this issue been fixed, or does it still require attention?

This issue will be closed in 15 days if no further activity occurs.

Thank you for your contributions.

[github-actions]

Hello 👋, to help manage issues we automatically close stale issues.

This issue has been automatically marked as stale because it has not had activity for quite some time.Has this issue been fixed, or does it still require attention?

This issue will be closed in 15 days if no further activity occurs.

Thank you for your contributions.