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.

}
        tryLogCurrentException(log);
        /** If exception is received from remote server, then stack trace is embedded in message.
          * If exception is thrown on local server, then stack trace is in separate field.
          */
        std::string exception_message = getCurrentExceptionMessage(with_stacktrace, true);
        int exception_code = getCurrentExceptionCode();
        trySendExceptionToClient(exception_message, exception_code, request, response, used_output);
    }
    used_output.finalize();
}
DynamicQueryHandler::DynamicQueryHandler(IServer & server_, const std::string & param_name_)
    : HTTPHandler(server_, "DynamicQueryHandler"), param_name(param_name_)
{
}
bool DynamicQueryHandler::customizeQueryParam(ContextMutablePtr context, const std::string & key, const std::string & value)
{
    if (key == param_name)
        return true;    
    if (startsWith(key, "param_"))
    {
        
        const String parameter_name = key.substr(strlen("param_"));
        if (!context->getQueryParameters().contains(parameter_name))
            context->setQueryParameter(parameter_name, value);
        return true;
    }
    return false;
}
std::string DynamicQueryHandler::getQuery(HTTPServerRequest & request, HTMLForm & params, ContextMutablePtr context)