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.

{
    if (likely(!startsWith(request.getContentType(), "multipart/form-data")))
    {
        
        
        
        std::string query_param = params.get(param_name, "");
        return query_param.empty() ? query_param : query_param + "\n";
    }
    
    
    ExternalTablesHandler handler(context, params);
    params.load(request, request.getStream(), handler);
    std::string full_query;
    
    for (const auto & it : params)
    {
        if (it.first == param_name)
        {
            full_query += it.second;
        }
        else
        {
            customizeQueryParam(context, it.first, it.second);
        }
    }
    return full_query;
}
PredefinedQueryHandler::PredefinedQueryHandler(