Add rxfire (split off from existing firebase-js-sdk)

[rwhogg]

Link to project (GitHub): https://github.com/firebaseextended/rxfire

Describe the project in 2-3 sentences: Firebase integration for RXJS

Why would Firebase developers be interested in this project?: Useful if you want to combine observable streams from more than one Firebase data source (eg. an image from Cloud Storage with a Firestore document)

Have you added a .opensource/project.json file to your site: No

It should be noted that rxfire is sort-of already included on this site: it was split off from the Firebase JS SDK, see https://firebaseopensource.com/projects/firebase/firebase-js-sdk/packages/rxfire/readme.md/