Add interop API to enumerate fallback folders without requiring a solution/project

[debonte]

Environment Details

NuGet product used: VS UI
NuGet version: 5.6.0.6489
VS version: 16.7
Worked before?: No

Feature Request

NuGet settings can be set in the following locations:

• machine-wide (%ProgramFiles(x86)%\NuGet\Config\*.config)
• user-specific (%APPDATA%\NuGet\NuGet.config)
• solution-wide

The XAML and WinForms teams need an API to enumerate machine-wide and user-specific fallback folders that:

1. Works without requiring a solution to be open
2. Ignores solution-wide settings if a solution is loaded

In order to call this API from within VS, it needs to be an interop API.

Likely choices to support from NuGet according to @rrelyea:

1. Tweak existing VS interop API to allow null value (or similar) to get what you need. Note that the requirement to ignore solution config when a solution is open may make this option impossible.
2. Add a new VS interop API.

[debonte]

@rrelyea @zivkan @aortiz-msft @mgoertz-msft

[nkolev92]

Docs issue: https://github.com/NuGet/docs.microsoft.com-nuget/issues/2084

@debonte PR ready.