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 (tryAddHeadersFromConfig(response, config))
    {
        response.setKeepAlive(false);
        response.setStatusAndReason(HTTPResponse::HTTP_NO_CONTENT);
        response.send();
    }
}
}
static String base64Decode(const String & encoded)
{
    String decoded;
    Poco::MemoryInputStream istr(encoded.data(), encoded.size());
    Poco::Base64Decoder decoder(istr);
    Poco::StreamCopier::copyToString(decoder, decoded);
    return decoded;
}
static String base64Encode(const String & decoded)
{
    std::ostringstream ostr; 
    ostr.exceptions(std::ios::failbit);
    Poco::Base64Encoder encoder(ostr);
    encoder.rdbuf()->setLineLength(0);
    encoder << decoded;
    encoder.close();
    return ostr.str();
}
static Poco::Net::HTTPResponse::HTTPStatus exceptionCodeToHTTPStatus(int exception_code)