Add ProblemDetails helpers to minimal actions

[pranavkm]

• Move ProblemDetails from MVC lower in to the ASP.NET Core stack (type forward)
• Add ProblemDetails as a helper method.

ValidationProblemDetails is tricky because it references ModelStateDictionary, so we'll leave it be in MVC for the immediate future.

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API Review:

• Type forward Microsoft.AspNetCore.Mvc.ProblemDetails to the Microsoft.AspNetCore.Http.Extensions assembly.

+ namespace Microsoft.AspNetCore.Http.Extensions
+ {
+     public class HttpValidationProblemDetails : ProblemDetails
+     {
+         public HttpValidationProblemDetails();
+         public HttpValidationProblemDetails(IDictionary<string, string[]> errors);
+         public IDictionary<string, string[]> Errors { get; }
+     }
+ }

namespace Microsoft.AspNetCore.Mvc
{
-   public class ValidationProblemDetails : ProblemDetails
+   public class ValidationProblemDetails : HttpValidationProblemDetails
    {
    }
}

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[pranavkm]

API review feedback - move the ValidationHttpProblemDetails to Microsoft.AspNetCore.Http.

- namespace Microsoft.AspNetCore.Http.Extensions
+ namespace Microsoft.AspNetCore.Http