add using System.Net.Http to GraphQLHttpWebSocket

Author: rose-a

src/GraphQL.Client/Websocket/GraphQLHttpWebSocket.cs

@@ -1,4 +1,8 @@
 using System.Diagnostics;
+#pragma warning disable IDE0005
+// see https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/implicit-global-using-netfx
+using System.Net.Http;
+#pragma warning restore IDE0005
 using System.Net.WebSockets;
 using System.Reactive;
 using System.Reactive.Disposables;