Add license info; implement IOpenApiReferenceable interface

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -1,14 +1,20 @@
-using System.Collections.Generic;
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. 
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Text.Json.Nodes;
 using Microsoft.OpenApi.Any;
 using Microsoft.OpenApi.Interfaces;
+using Microsoft.OpenApi.Writers;
 
 namespace Microsoft.OpenApi.Models
 {
     /// <summary>
     /// The Schema Object allows the definition of input and output data types.
     /// </summary>
-    public class OpenApiSchema : IOpenApiExtensible
+    public class OpenApiSchema : IOpenApiExtensible, IOpenApiReferenceable
     {
         /// <summary>
         /// Follow JSON Schema definition. Short text providing information about the data.