Move UseMiddleware to Http.Abstractions

[davidfowl]

It's a pretty core overload to creating middleware and it would be great if we could move it to the main assembly. We want to make a middleware template in VS that does the following:

using Microsoft.AspNet.Builder;
using Microsoft.AspNet.Http;

namespace ProjectNs
{
    public static class Middleware1Extensions
    {
        public IApplicationBuilder UseMiddleware1(IApplicationBuilder app)
        {
            app.UseMiddleware<Middleware1>();
        }
    }

    public class Middleware1
    {
        private readonly RequestDelegate _next;
        public Middleware1(RequestDelegate next)
        {
            _next = next;
        }

        public Task Invoke(HttpContext context)
        {
             return _next(context);
        }
    }
}

Item templates can't add packages so we need to move the UseMiddleware to the core assembly. I'm not comfortable with making our core Http types rely on DI so maybe we make that thing shared source (just a piece of ActivatorUtilities we need). The other option is to change the template but we want to encourage DI by default. For sake of options:

using Microsoft.AspNet.Builder;
using Microsoft.AspNet.Http;

namespace ProjectNs
{
    public static class Middleware1Extensions
    {
        public IApplicationBuilder UseMiddleware1(IApplicationBuilder app)
        {
            app.Use(next => new Middleware1(next).Invoke);
        }
    }

    public class Middleware1
    {
        private readonly RequestDelegate _next;
        public Middleware1(RequestDelegate next)
        {
            _next = next;
        }

        public Task Invoke(HttpContext context)
        {
             return _next(context);
        }
    }
}

[davidfowl]

/cc @Tratcher @DamianEdwards @muratg

We want to do this for beta5 so we can add the item template to VS.

[Tratcher]

The new dependency would be on DI.Abstractions, not DI, so that's not terrible.

[davidfowl]

Thats pretty terrible.

[Tratcher]

@rustd