Add project: Firebase Functions Downloader

**Link to project (GitHub)**: https://github.com/omar-dulaimi/firebase-functions-downloader

**Describe the project in 2-3 sentences**: Downloads Cloud Functions code

**Why would Firebase developers be interested in this project?**:
Sometimes projects don't start with version control from day one, or improperly use it. In that case code will be scattered across team members and nobody would have all the code. So to help these teams get their already deployed code back to their machines, this project came to existence.

**Have you added a `.opensource/project.json` file to your site**: Yes

Start by checking the linked Firebase Functions Downloader repository and its .opensource/project.json file, then trace how submitted projects are added to firebaseopensource.com. Add the project using the site's existing submission path and verify that its listing appears with the supplied link and description.

Written by the indexing model from the issue text.