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.

const std::string & s, int exception_code, HTTPServerRequest & request, HTTPServerResponse & response, Output & used_output)
try
{
    response.set("X-ClickHouse-Exception-Code", toString<int>(exception_code));
    
    
    
    if (request.getMethod() == Poco::Net::HTTPRequest::HTTP_POST
        && response.getKeepAlive()
        && exception_code != ErrorCodes::HTTP_LENGTH_REQUIRED
        && !request.getStream().eof())
    {
        request.getStream().ignoreAll();
    }
    if (exception_code == ErrorCodes::REQUIRED_PASSWORD)
    {
        response.requireAuthentication("ClickHouse server HTTP API");
    }
    else
    {
        response.setStatusAndReason(exceptionCodeToHTTPStatus(exception_code));
    }
    if (!response.sent() && !used_output.out_maybe_compressed)
    {