Testing Subscriptions with Microsoft.AspNetCore.Mvc.Testing

`GraphQLHttpClient` works really well for writing integration tests for queries and mutations:

```csharp
var client = _factory.CreateClient();
var opts = new GraphQLHttpClientOptions { EndPoint = new Uri(client.BaseAddress!, "graphql") };

Client = new GraphQLHttpClient(opts, new SystemTextJsonSerializer(), client);
```

However it falls short on subscriptions. `GraphQLHttpWebSocket` is internal and does not seem to accept an injected socket or a factory so using it with `_factory.Server.CreateWebSocketClient()` isn't feasible.

It would be great if this were possible