[FR] Make Firebase UPM packages available on Github

### [READ] Guidelines

When filing a feature request please make sure the issue title starts with "[FR]".

A good feature request ideally
* is either immediately obvious (i.e. "Add Sign in with Apple support"), or
* starts with a use case that is not achievable with the existing Firebase API and
includes an API proposal that would make the use case possible. The proposed API
change does not need to be very specific.

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### Feature proposal

* Firebase Component: All

Hi there! It would be nice if we could have the built SDKs hosted so they can be automatically updated with the Unity Package Manager, then rather than manually updating Tarballs it'd be a simple one click update. OpenUPM has an unofficial package but I'd rather not rely on a 3rd party for something as mission critical as Firebase so an officially hosted package would be great! Thank you!