Add response trailer feature

src/Microsoft.AspNetCore.Http.Features/IHttpResponseTrailersFeature.cs

@@ -0,0 +1,30 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.Http.Features
+{
+    public interface IHttpResponseTrailersFeature
+    {
+        /// <summary>
+        /// The response trailers to send.
+        /// </summary>
+        IHeaderDictionary Trailers { get; set; }
+
+        /// <summary>
+        /// Indicates if the server has started writing the response trailers.
+        /// If true, the <see cref="Trailers"/> are now immutable, and <see cref="OnStarting"/> should no longer be called.
+        /// </summary>
+        bool HasStarted { get; }
+
+        /// <summary>
+        /// Registers a callback to be invoked just before writing the response trailers.
+        /// This is the last chance to modify the <see cref="Trailers"/>.
+        /// </summary>
+        /// <param name="callback">The callback to invoke before writing the trailers.</param>
+        /// <param name="state">The state to pass into the callback.</param>
+        void OnStarting(Func<object, Task> callback, object state);
+    }
+}

src/Microsoft.AspNetCore.Http/Features/HttpResponseTrailersFeature.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.Http.Features
+{
+    public class HttpResponseTrailersFeature : IHttpResponseTrailersFeature
+    {
+        public HttpResponseTrailersFeature()
+        {
+            Trailers = new HeaderDictionary();
+        }
+
+        public IHeaderDictionary Trailers { get; set; }
+
+        public virtual bool HasStarted => false;
+
+        public virtual void OnStarting(Func<object, Task> callback, object state)
+        {
+        }
+    }
+}