Consider how to solve the credential plugin + proxy scenario

[nkolev92]

NuGet itself has some very primitive proxy support.

https://docs.microsoft.com/en-us/nuget/reference/nuget-config-file

In the past, the old credential provider did not have any proxy support.

There are 2 options here:

• NuGet passes the proxy settings at/sometime after the initialization
• The Credential provider reads the NuGet config for the proxy settings.

Need to analyze the pros and cons for each.

//cc @keithrob @pspill @rrelyea

[nkolev92]

closed in NuGet/NuGet.Client#2250