Serialize query response directly to Servlet's output stream to avoid intermediate String object creation

[shailender-bathula]

I checked this improvement using this test customers query shailender-bathula/samples@fd1bf6d and bombardier HTTP benchmarking tool.

I ran the application using the default settings i.e. ../gradlew appRun and benchmarking like this:
bombardier --method POST --header "Content-Type: application/json" --duration=70s --connections=110 --body '{ "query": "{ customers {id firstName lastName yearOfBirth email} }" }' http://localhost:8080/servlet-hello-world/graphql

There was an improvement of 5 - 10 requests/second before and after in those tests. The difference may be more in higher throughput environments.