I want to add a `predefined_query_handler` to serve a single JSON from my query.
For example

```xml
<rule>
      <url>/my/url</url>
      <methods>GET</methods>
      <handler>
        <type>predefined_query_handler</type>
        <query>select '{"my": "json"}' format JSONAsString</query>
      </handler>
    </rule>
```

I can use `RawBLOB` but it sets up Content-Type 'text/plain' instead of 'application/json'.
I think JSONAsString should also work with output.

response.set("WWW-Authenticate", "Basic");
        else
            response.set("WWW-Authenticate", "Basic realm=\"" + required_credentials.getRealm() + "\"");
        response.setStatusAndReason(HTTPResponse::HTTP_UNAUTHORIZED);
        response.send();
        return false;
    }
    catch (const Authentication::Require<GSSAcceptorContext> & required_credentials)
    {
        request_credentials = server.context()->makeGSSAcceptorContext();
        if (required_credentials.getRealm().empty())
            response.set("WWW-Authenticate", "Negotiate");
        else
            response.set("WWW-Authenticate", "Negotiate realm=\"" + required_credentials.getRealm() + "\"");
        response.setStatusAndReason(HTTPResponse::HTTP_UNAUTHORIZED);
        response.send();
        return false;
    }
    request_credentials.reset();
    return true;
}
void HTTPHandler::processQuery(
    HTTPServerRequest & request,
    HTMLForm & params,
    HTTPServerResponse & response,
    Output & used_output,
    std::optional<CurrentThread::QueryScope> & query_scope)
{
    using namespace Poco::Net;
    LOG_TRACE(log, "Request URI: {}", request.getURI());