Adding support for query cancellation

Query cancellation support is added via CancellationToken mechanisms that
were implemented previously. This change adds a new request type
"query/cancel" that will issue the cancellation token. Unit tests were
also added.

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/Contracts/QueryCancelRequest.cs

@@ -0,0 +1,36 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.ServiceLayer.Hosting.Protocol.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts
+{
+    /// <summary>
+    /// Parameters for the query cancellation request
+    /// </summary>
+    public class QueryCancelParams
+    {
+        public string OwnerUri { get; set; }
+    }
+
+    /// <summary>
+    /// Parameters to return as the result of a query dispose request
+    /// </summary>
+    public class QueryCancelResult
+    {
+        /// <summary>
+        /// Any error messages that occurred during disposing the result set. Optional, can be set
+        /// to null if there were no errors.
+        /// </summary>
+        public string Messages { get; set; }
+    }
+
+    public class QueryCancelRequest
+    {
+        public static readonly 
+            RequestType<QueryCancelParams, QueryCancelResult> Type =
+            RequestType<QueryCancelParams, QueryCancelResult>.Create("query/cancel");
+    }
+}