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

**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/