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.

#include <Common/StringUtils/StringUtils.h>
#include <Common/setThreadName.h>
#include <Common/typeid_cast.h>
#include <base/getFQDNOrHostName.h>
#include <base/scope_guard.h>
#include <Server/HTTP/HTTPResponse.h>
#include <Common/config.h>
#include <Poco/Base64Decoder.h>
#include <Poco/Base64Encoder.h>
#include <Poco/Net/HTTPBasicCredentials.h>
#include <Poco/Net/HTTPStream.h>
#include <Poco/MemoryStream.h>
#include <Poco/StreamCopier.h>
#include <Poco/String.h>
#include <chrono>
#include <sstream>
#if USE_SSL
#include <Poco/Net/X509Certificate.h>
#endif
namespace DB
{
namespace ErrorCodes
{
    extern const int LOGICAL_ERROR;
    extern const int CANNOT_PARSE_TEXT;
    extern const int CANNOT_PARSE_ESCAPE_SEQUENCE;
    extern const int CANNOT_PARSE_QUOTED_STRING;
    extern const int CANNOT_PARSE_DATE;
    extern const int CANNOT_PARSE_DATETIME;
    extern const int CANNOT_PARSE_NUMBER;