API review updates for CORS

[Eilon]

In particular, a lot of types need to move out of the Cors.Core namespace. The main Cors namespace should have the most commonly used types, and plumbing can go into a sub-namespace.

[Eilon]

Met with @kichalla @rynowak and @danroth27 with these conclusions:

• Merge both packages into Microsoft.AspNet.Cors.
  • The current package separation in practice provided very little value, and unfortunately was self-defeating with respect to the original purpose of having two packages: to encourage developers using MVC to use just the parts MVC needed. Unfortunately it was most common to reference the both packages transitively anyway (which was harmless, but unnecessary)
• Move DisableCorsAttribute and EnableCorsAttribute into .Cors namespace
• Move all other types from both original assemblies (including CorsMiddleware and CorsPolicyBuilder) into .Infrastructure namespace.
• Post announcement that links back to this issue

[davidfowl]

Nice