[FR]: Unity Package Manager git support (again)

### Description

A lot of Google Unity Packages are already (and again) available as a Unity Package Manger git package, like the Google Play Plugins, Google Mobile Ads, etc.

It would just make the integration smoother and also reduces overall footprints on the internet (as i.e. the packages don't need to be checked in) and only downloaded and cached if needed by Unity.

### API Proposal

_No response_

### Firebase Product(s)

All

### Targeted Platform(s)

Apple Platforms, Android, Desktop