Allow specifying the databaseId for the firestore database

[daveleroy]

Firebase now allows creating multiple firestore databases. It would be nice to be able to select which database to store customer information/events in via a configuration option.

export declare function getFirestore(databaseId: string): Firestore;

https://firebase.google.com/docs/reference/admin/node/firebase-admin.firestore.md#getfirestore_53dc891