Builtin NuGet client should support credential providers

nuget.exe supports using external credential providers. These are used for example for accessing Azure Artifacts feeds with the CredentialProvider.VSS.exe.

This mostly works for the NuGet* commands (except for #2345), but not for the NuGet internal client (eg. for #load/#addin directives)

https://docs.microsoft.com/en-us/azure/devops/artifacts/nuget/nuget-exe?view=vsts&tabs=new-nav#advanced-credential-provider-scenarios

See https://docs.microsoft.com/en-au/nuget/reference/extensibility/nuget-exe-credential-providers for details of how nuget.exe looks for credential providers (including the NUGET_CREDENTIALPROVIDERS_PATH environment variable)