private void beginJsonCallback(OutputStream stream) throws IOException {
        if (shouldRenderJsonCallback()) {
            String jsonCallback = getJsonCallback() + "(";
            stream.write(jsonCallback.getBytes(StandardCharsets.UTF_8));
            this.stream = stream;
        }
    }
Adds JSONP (Json with padding) support.

Basically, if the JSON renderer receives a query parameter "jsoncallback=...",
the JSON response will be wrapped in a function call with the name specified
by the client. This side-steps the same-origin policy, thus supports calling
Vespa from javascript loaded from a different domain then the Vespa instance.
container-search/src/main/java/com/yahoo/search/rendering/JsonRenderer.java