Allow custom handling of antiforgery failures

[poke]

This fixes the problem highlighted in dotnet/aspnetcore#3616 and adds support to allow custom handling of antiforgery validation failures. Right now, the ValidateAntiforgeryTokenAuthorizationFilter simply sets a BadRequestResult and there is no way to hook into this behavior and do anything else.

This introduces a new AntiforgeryValidationFailedResult that extends BadRequestResult but allows to be identified explicitly within always-running result filters (they need to be always-running in order to go around the short-circuiting behavior of the MVC filter pipeline).

An example for a filter that intercepts the result is included in the functional test and essentially looks like this:

public class RedirectAntiforgeryValidationFailedResultFilter : IAsyncAlwaysRunResultFilter
{
    public Task OnResultExecutionAsync(ResultExecutingContext context, ResultExecutionDelegate next)
    {
        if (context.Result is AntiforgeryValidationFailedResult result)
        {
            context.Result = new RedirectResult("http://example.com/antiforgery-redirect");
        }

        return next();
    }
}

When applied in the pipeline, this filter will intercept the AntiforgeryValidationFailedResult and replace it with something else.

[poke]

Updated the PR for way better approach. I like this :) Thanks @pranavkm for your feedback!

[pranavkm]

@rynowak \ @javiercn any concerns?

[pranavkm]

👍

[rynowak]

This is cool!

[pranavkm]

@dotnet-bot build OSX 10.12 Release Build please

[pranavkm]

Thanks for the PR!

[poke]

Thanks for the quick merge, and the feedback! 😊