Move some WebHostBuilderExtensions to Hosting.Abstractions

[Tratcher]

IISIntegration was able to depend just on Hosting.Abstractions until it called CaptureStartupErrors.

Move extensions that may be used by extensions from other packages and not just app code. E.g:

• UseConfiguration
• CaptureStartupErrors
• UseEnvironment
• UseUrls
• Start

Don't move:

• UseStartup
• UseDefaultHostingConfiguration
• UseServer? - What if I wanted a UseKestrel extension?
• UseContentRoot
• UseWebRoot

https://github.com/aspnet/Hosting/blob/dev/src/Microsoft.AspNetCore.Hosting/WebHostBuilderExtensions.cs

@JunTaoLuo @davidfowl

[davidfowl]

Why not move everything that can be moved?

[Tratcher]

Doesn't matter, as long as it doesn't require too much implementation code or any dependencies.