[FR] add listDatabases() to admin SDK

**Is your feature request related to a problem? Please describe.**
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I'm building an app that connects to user's firestore database and I have no way of fetching the databases they already use in their project. listCollections() is a method that comes in very handy to do something similar on a lower level.

**Describe the solution you'd like**
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listDatabases() should return a list of the databases for a certain db object that corresponds to a firestore project.

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Maybe there's a way I can get this to work in my node environment with the cli command `firebase firestore:databases:list` but not sure how...

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