Schema compare cancel operation (#826)

* First cut of schema compare cancel (private nuget)

* Update Dacfx nuget to a published version

src/Microsoft.SqlTools.ServiceLayer/SchemaCompare/Contracts/SchemaCompareCancellationRequest.cs

@@ -0,0 +1,30 @@
+//
+// 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.Hosting.Protocol.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Utility;
+
+namespace Microsoft.SqlTools.ServiceLayer.SchemaCompare.Contracts
+{
+    /// <summary>
+    /// Parameters for a schema compare cancel request
+    /// </summary>
+    public class SchemaCompareCancelParams
+    {
+        /// <summary>
+        /// Operation id of the schema compare operation
+        /// </summary>
+        public string OperationId { get; set; }
+    }
+
+    /// <summary>
+    /// Defines the Schema Compare cancel comparison request type
+    /// </summary>
+    class SchemaCompareCancellationRequest
+    {
+        public static readonly RequestType<SchemaCompareCancelParams, ResultStatus> Type =
+            RequestType<SchemaCompareCancelParams, ResultStatus>.Create("schemaCompare/cancel");
+    }
+}