[FR] add listDatabases() to admin SDK

[davidoort]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] 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 A clear and concise description of what you want to happen. listDatabases() should return a list of the databases for a certain db object that corresponds to a firestore project.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered. 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...

Additional context Add any other context, code samples or screenshots about the feature request here.