Add .NET Core 3.1 target

[wrall]

When trying to use the Microsoft.Azure.DataLake.Store package from a .NET Core 3.1+ application that specifically targets win10-x64, the package pulls in the Microsoft.NETCore.App package dependency, which tries to use an old version of dotnet.exe and other framework assemblies. In order to use this package from .NET Core 3.1+, we need to add some explicit things to block pulling those assemblies. Having an explicit .NET Core 3.1 target in a new package version would help prevent this.

[tkreindler]

I'd just like to add a +1 to this PR, this simple fix could really help solve our package dependency issues.

[akharit]

@wrall I believe #60 should fix this. I'll update when that gets merged, and a new version is released.