Switch from nuspecs to csproj to generate nuget packages #790

Summary

Currently we have a bunch of nuspecs to generate nuget packages. With sdk style project its also possible to do this from the csproj.

https://docs.microsoft.com/en-us/dotnet/core/tools/csproj#nuget-metadata-properties

Analysis

• Automatically figures out dependencies and versions. For example when you have a project reference to a project with version 1.4.3 it will also be added to the nuget package.
• Enables the use of dotnet pack
• Easier multi targeting
• Generally cleaner and less duplicate data.
• Consider doing this after all projects are migrated to sdk style projects