Preliminary desktop support (netstandard2.0,net6,net framework 4.8)

[Jon2G]

Add support for desktop environments with: https://www.nuget.org/packages/Google.Cloud.Firestore.V1 https://www.nuget.org/packages/Google.Cloud.Firestore

[angelru]

@Jon2G thanks for this, it seems that this package is no longer maintained, there is another one that seems to be in use and active.

https://github.com/TobiasBuchholz/Plugin.Firebase