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.

#pragma once
#include <Core/Names.h>
#include <Server/HTTP/HTMLForm.h>
#include <Server/HTTP/HTTPRequestHandler.h>
#include <Server/HTTP/WriteBufferFromHTTPServerResponse.h>
#include <Common/CurrentMetrics.h>
#include <Common/CurrentThread.h>
#include <re2/re2.h>
namespace CurrentMetrics
{
    extern const Metric HTTPConnection;
}
namespace Poco { class Logger; }
namespace DB
{
class Session;
class Credentials;
class IServer;
struct Settings;
class WriteBufferFromHTTPServerResponse;