Use trimmed packages to speedup runner updates (#1568)

* consume trimmed packages.

* .

* .

* .

* .

src/Sdk/DTWebApi/WebApi/PackageMetadata.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.Runtime.Serialization;
 
@@ -110,5 +111,43 @@ namespace GitHub.DistributedTask.WebApi
             get;
             set;
         }
+
+        /// <summary>
+        /// A set of trimmed down packages:
+        /// - the package without 'externals'
+        /// - the package without 'dotnet runtime'
+        /// - the package without 'dotnet runtime' and 'externals'
+        /// </summary>
+        [DataMember(EmitDefaultValue = false)]
+        public List<TrimmedPackageMetadata> TrimmedPackages
+        {
+            get;
+            set;
+        }
+    }
+
+    [DataContract]
+    public class TrimmedPackageMetadata
+    {
+        [DataMember(EmitDefaultValue = false)]
+        public string HashValue { get; set; }
+
+        [DataMember(EmitDefaultValue = false)]
+        public string DownloadUrl { get; set; }
+
+        public Dictionary<string, string> TrimmedContents
+        {
+            get
+            {
+                if (m_trimmedContents == null)
+                {
+                    m_trimmedContents = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                }
+                return m_trimmedContents;
+            }
+        }
+
+        [DataMember(Name = "TrimmedContents", EmitDefaultValue = false)]
+        private Dictionary<string, string> m_trimmedContents;
     }
 }