New kestrel configuration & related APIs

[Tratcher]

Kestrel now has first class support for loading endpoints via config. See aspnet/KestrelHttpServer#2186

[Tratcher]

@shirhatti @danroth27 We'll want to have a blog post for this in preview1. We can use that as the rough draft for the docs.

[Rick-Anderson]

@Tratcher give us a change to edit before you publish.

[danroth27]

The new scheme is covered in the ASP.NET Core 2.1 roadmap blog post is here: https://blogs.msdn.microsoft.com/webdev/2018/02/02/asp-net-core-2-1-roadmap/#security

[scottaddie]

@guardrex Any interest in taking this one?