[WIP] GH2028: adding credential support for inprocess nuget client (please don't merge)

[Insire]

added NuGet.Credentials package,
added WIP call to PluginCredentialProviderBuilder

relates to #2028

[devlead]

@Insire there's a conflict with this PR and develop that needs to be resolved before this PR can be reviewed.

[axdiamond]

@Insire do you intend to finish this PR?

[Insire]

RL is rather busy for me at the moment, so if anyone wants to take over, go ahead.

[bjorkstromm]

@Insire I'll take over at some point. Talked to NuGet team today, and we should dig into the plugin system for authentication.

[Insire]

Sounds good.

[gep13]

@bjorkstromm can you elaborate on this comment:

Talked to NuGet team today, and we should dig into the plugin system for authentication.

Is there something specific that we should be looking at here? If so, can you point us in the right direction? Or perhaps update the original issue with your thoughts on this?

Is this something that should be left to say the "NuGet Authenticate" task in Azure DevOps (https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/package/nuget-authenticate?view=azure-devops) rather than something that should be brought into Cake?