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 (request.getVersion() == HTTPServerRequest::HTTP_1_1)
            response.setChunkedTransferEncoding(true);
        HTMLForm params(default_settings, request);
        with_stacktrace = params.getParsed<bool>("stacktrace", false);
        
        
        if (request.getMethod() == HTTPRequest::HTTP_POST && !request.getChunkedTransferEncoding() && !request.hasContentLength())
        {
            throw Exception(
                "The Transfer-Encoding is not chunked and there is no Content-Length header for POST request",
                ErrorCodes::HTTP_LENGTH_REQUIRED);
        }
        processQuery(request, params, response, used_output, query_scope);
        if (request_credentials)
            LOG_DEBUG(log, "Authentication in progress...");
        else
            LOG_DEBUG(log, "Done processing query");
    }
    catch (...)
    {
        SCOPE_EXIT({
            request_credentials.reset(); 
        });
        
        
        
        if (used_output.isFinalized())
        {
            tryLogCurrentException(log, "Cannot flush data to client");
            return;