Add license header

Author: MaggieKimani1

src/Microsoft.OpenApi.Tool/OpenApiService.cs

@@ -1,4 +1,7 @@
-using System;
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System;
 using System.IO;
 using System.Linq;
 using System.Net;

src/Microsoft.OpenApi.Tool/Program.cs

@@ -1,4 +1,7 @@
-using System.CommandLine;
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System.CommandLine;
 using System.CommandLine.Invocation;
 using System.IO;
 using System.Threading.Tasks;