[new Feature] Load FlutterUnityIntegration package via git URL in package manager

[mathtasatlime]

Hi,
a really nice feature to keep track of the exact version which is integrated in my unity project is to be able to load the FlutterUnityPackage directly via the package manager via git URL.

In case there is already a way for that let me know :)

[juicycleff]

Hmmm, that's good feedback. I Will add it to the pipeline