Add necessary usings

Author: MaggieKimani1

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
@@ -7,8 +7,12 @@
 using System.Net;
 using System.Net.Http;
 using System.Text;
+using System.Threading.Tasks;
+using System.Xml.Linq;
+using Microsoft.OData.Edm.Csdl;
 using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.OData;
 using Microsoft.OpenApi.Readers;
 using Microsoft.OpenApi.Services;
 using Microsoft.OpenApi.Validations;