Added multiple Firestore databases support

[KarenjanM]

Hey there! I saw an issue, where was discussed adding support for new feature ( Multiple database in a single project ) Here's a link to it: https://github.com/firebase/firebase-admin-python/issues/718

I've tested my solution and it works, would be happy if it could be added to the main package

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[lahirumaramba]

Duplicate of #719

[KarenjanM]

Duplicate of #719

actually it's not quite a duplicate, because the order of arguments in function get_app_service in 719 is not right and breaks if you try to launch that. The new argument options should be as last argument