Build PDMFramework package to target .NET Framework 4.5.1 - 4.8

[AmenJlili]

Situation:
We're currently only targeting .NET 472. As more and more people start using the framework, we'd probably need to include more builds of PDMFramework for 4.5.1 - 4.8 in our NuGet package.

Issue:
Our current NuGet packaging process uses nupack (vs extension to build nuget packages) which is limited. We need to create a script :

• Build PDMFramework for the version of .NET, we'd like to support
• Copy those dlls to lib folder (example 451 dll is copied to lib/net451 and so on) where the nuspec file exists.
• invoke the nuget pack on the package.nuspec to build the package.

A code review might be warranted because some things we are using for 472 (Tuples) are not available for lower versions of .NET and we need to add some dependencies (Example: Tuples)

[AmenJlili]

I'm holding off on implementing this for now. We're building PDMFramework to target 461 unless we got someone to work on this...