[FR] Please provide firebase package with UPM git url

### Feature proposal

With Game Package Registry by Google was shut down as of #1028. We understand the complication but the UPM system to include package still allow direct git dependency

With that, Google should create git repo for firebase package in github, maybe making a `Release` folder in this repo directly, and allow us to include all firebase package with this method in addition to `.tgz`

FYI, UPM has feature to include with path for pointing to specific subfolder https://docs.unity3d.com/Manual/upm-git.html#subfolder so we could only have one repo for all firebase module and point it to separate subfolder