feat: Expose databaseId getter on Firestore class

[IchordeDionysos]

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Fixes #1936

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[tom-andersen]

This seems like a reasonable and simple change. However, since this does affect the API surface, I will need to have this approved internally.

Hang on while I handle our internal process for these types of changes.

[tom-andersen]

Hey. The internal API review is ok with exposing databaseId, but projectId was asked to take a different approach.

If you restrict change to just exposing databaseId, I will merge right away.

[IchordeDionysos]

@tom-andersen changed

[IchordeDionysos]

@tom-andersen changed