Add framework specific environment variables for plugin discovery

[nkolev92]

Bug

Fixes: NuGet/Home#8151
Regression: No

• Last working version:
• How are we preventing it in future:

Fix

Details: Replaces https://github.com/NuGet/NuGet.Client/pull/2859/files.

NUGET_PLUGIN_PATHS can't be configured statically, to work for both nuget.exe and dotnet. If you add paths for the .exe and .dll credential providers to handle both cases, nuget fails trying to execute a dll and dotnet fails trying to execute the full framework .exe.

A few approaches were considered:

• Extend on the convention based discovery, as described in previous PR and issue.
  Basically prefer .dll for netcore and .exe for netfx.
  The concern with this approach is dotnet core 3.0 SDK creates exes by default for console netcoreapp3.0 projects. Another concern is eliminating the ability to disable all plugins by specifying " ".

• Add 2 new framework specific environment variables. Those take precedence over the generic variable.

The 2nd is the approach we are implementing.

Docs changes: NuGet/docs.microsoft.com-nuget#1599

//cc @zjrunner @zarenner

Testing/Validation

Tests Added: Yes
Reason for not adding tests:
Validation:

[zarenner]

Looks great, excited to have this. Thanks Nikolche.

[zivkan]

I still think it would be good to change the #if to a runtime check now, rather than later when we try to reduce our TFMs.