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.

CompressionMethod http_response_compression_method = CompressionMethod::None;
    if (!http_response_compression_methods.empty())
    {
        
        
        
        if (std::string::npos != http_response_compression_methods.find("br"))
            http_response_compression_method = CompressionMethod::Brotli;
        else if (std::string::npos != http_response_compression_methods.find("gzip"))
            http_response_compression_method = CompressionMethod::Gzip;
        else if (std::string::npos != http_response_compression_methods.find("deflate"))
            http_response_compression_method = CompressionMethod::Zlib;
        else if (std::string::npos != http_response_compression_methods.find("xz"))
            http_response_compression_method = CompressionMethod::Xz;
        else if (std::string::npos != http_response_compression_methods.find("zstd"))
            http_response_compression_method = CompressionMethod::Zstd;
    }
    bool client_supports_http_compression = http_response_compression_method != CompressionMethod::None;
    
    
    bool internal_compression = params.getParsed<bool>("compress", false);
    
    size_t buffer_size_total = std::max(
        params.getParsed<size_t>("buffer_size", DBMS_DEFAULT_BUFFER_SIZE), static_cast<size_t>(DBMS_DEFAULT_BUFFER_SIZE));
    
    
    bool buffer_until_eof = params.getParsed<bool>("wait_end_of_query", false);
    size_t buffer_size_http = DBMS_DEFAULT_BUFFER_SIZE;
    size_t buffer_size_memory = (buffer_size_total > buffer_size_http) ? buffer_size_total : 0;
    unsigned keep_alive_timeout = config.getUInt("keep_alive_timeout", 10);