Adds new graph comparison request handler to the Execution Plan Service. (#1438)

* Adds new graph comparison request handler to the Execution Plan Service.

* Code review changes

* Renames execution plan compare params, result, and request classes.

src/Microsoft.SqlTools.ServiceLayer/ExecutionPlan/Contracts/ExecutionGraphComparisonResult.cs

@@ -0,0 +1,43 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.Collections.Generic;
+
+namespace Microsoft.SqlTools.ServiceLayer.ExecutionPlan.Contracts
+{
+    /// <summary>
+    /// An ExecutionGraphComparisonResult is composed of an execution plan node, but has additional properties
+    /// to keep track of matching ExecutionGraphComparisonResult nodes for execution plan nodes present in the
+    /// the graph being compared against. This class also features a group index that can assist
+    /// with coloring similar sections of execution plans in the UI.
+    /// </summary>
+    public class ExecutionGraphComparisonResult
+    {
+        /// <summary>
+        /// The base ExecutionPlanNode for the ExecutionGraphComparisonResult.
+        /// </summary>
+        public ExecutionPlanNode BaseNode { get; set; }
+        /// <summary>
+        /// The children of the ExecutionGraphComparisonResult.
+        /// </summary>
+        public List<ExecutionGraphComparisonResult> Children { get; set; } = new List<ExecutionGraphComparisonResult>();
+        /// <summary>
+        /// The group index of the ExecutionGraphComparisonResult.
+        /// </summary>
+        public int GroupIndex { get; set; }
+        /// <summary>
+        /// Flag to indicate if the ExecutionGraphComparisonResult has a matching node in the compared execution plan.
+        /// </summary>
+        public bool HasMatch { get; set; }
+        /// <summary>
+        /// List of matching nodes for the ExecutionGraphComparisonResult.
+        /// </summary>
+        public List<ExecutionGraphComparisonResult> MatchingNodes { get; set; } = new List<ExecutionGraphComparisonResult>();
+        /// <summary>
+        /// The parent of the ExecutionGraphComparisonResult.
+        /// </summary>
+        public ExecutionGraphComparisonResult ParentNode { get; set; }
+    }
+}