Use `dotnet pack` against a csproj to create nuget package

[eerhardt]

We are currently using nuget.exe and a .nuspec file to create the Spark NuGet package. We should instead use dotnet pack and MSBuild to do this.

The advantages are that we can:

1. Maintain the dependencies in a single place (the .csproj).
2. Use MSBuild properties to control things like the version number. This allows for pre-release daily builds to have different version numbers.
3. It works cross-platform
4. We can take advantage of new features, like .snupkg packages for symbols, source link, and the <repository /> tag in the .nuspec which tells users which git repo and commit this package was built from.

@imback82 @safern @rapoth