Upgrade to target .NET Standard

[mgurlitz]

• Converted main library to support .NET Framework 4.6 + Standard 2.1
• Converted tests/samples to run with .NET Core 3.1
• Removed UWP support, since Standard 2.0 works with it and 2.1 is "TBD"
• Replaced UWP-only credentials library with an alternative from NuGet
• Replaced partial classes used for testing with internal mockable interfaces
• Fixed Xbox deployment test bug related to mock setup

[mgurlitz]

Yeah, the unit tests all pass, and I verified the sample programs work with both a PC and Xbox devkit, thank you! The only thing I haven't tried running is the MockDataGenerator.