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.

in_post_maybe_compressed = std::move(in_post);
    std::unique_ptr<ReadBuffer> in;
    static const NameSet reserved_param_names{"compress", "decompress", "user", "password", "quota_key", "query_id", "stacktrace",
        "buffer_size", "wait_end_of_query", "session_id", "session_timeout", "session_check"};
    Names reserved_param_suffixes;
    auto param_could_be_skipped = [&] (const String & name)
    {
        
        if (name.empty())
            return true;
        if (reserved_param_names.contains(name))
            return true;
        for (const String & suffix : reserved_param_suffixes)
        {
            if (endsWith(name, suffix))
                return true;
        }
        return false;
    };
    
    
    
    
    
    
    
    
    
    const auto & settings = context->getSettingsRef();