[Feature] Firebase Functions support

The Firestore plugin only connects to the Firestore service of Firebase. Being able to connect to Firebase Functions and invoke functions from an Appsmith app would mean a much better integration with the Firebase ecosystem.

Additionally, since credentials would be the same for Firestore and Functions within a single Firebase project, a nice quality-of-life feature would be to not have to duplicate the credential information, once for Firestore and once again for Functions support.