Make AbstractGraphQLHttpServlet to be more customisable

[drupalspring]

Maybe related to #182 .

Currently , all of the request handling logic are implemented in the getHandler and postHandler , but all of its internal methods are private methods which make the subclass cannot customise the request handling logic by override the corresponding methods.

My use-case is that I would like to override the logic of setting the HttpServletResponse (e.g its status code , the content write to the outputStream)(i.e the following codes in query() )

        if (!(result.getData() instanceof Publisher)) {
            resp.setContentType(APPLICATION_JSON_UTF8);
            resp.setStatus(STATUS_OK);
            resp.getWriter().write(graphQLObjectMapper.serializeResultAsJson(result));
        }

I am thinking perhaps to refactor the logic of getHandler/postHandler into many sensible protected methods such that subclass can override it
(e.g queryIntroSpectionRequest() , setResponse() .....)

[oliemansm]

The refactoring that needs to be done is a lot more extensive. The AbtractGraphQLHttpServlet has way too much responsibilities at the moment. Make sure you keep an eye on the PR for #182 once that work is being done to see if it suits your needs too.