Serviceable package marker

[victorhurdugaci]

We need a way to specify that a package is serviceable. The feature is needed because we only leave breadcrumbs for the marked packages

PR for breadcrumbs: #1078

[victorhurdugaci]

@davidfowl I'm assigning this to you since you mentioned that you are going to talk to the NuGet team

[glennc]

I think we are going to pivot on how we do breadcrumbs. Come talk to me about it.

[muratg]

@victorhurdugaci let's get this prioritized. Fundamentals team has started testing these things already. :)

[victorhurdugaci]

@ChengTian please resolve this when you merge the serviceable changes