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.

: server(server_)
    , log(&Poco::Logger::get(name))
    , default_settings(server.context()->getSettingsRef())
{
    server_display_name = server.config().getString("display_name", getFQDNOrHostName());
}
HTTPHandler::~HTTPHandler() = default;
bool HTTPHandler::authenticateUser(
    HTTPServerRequest & request,
    HTMLForm & params,
    HTTPServerResponse & response)
{
    using namespace Poco::Net;
    
    
    std::string user = request.get("X-ClickHouse-User", "");
    std::string password = request.get("X-ClickHouse-Key", "");
    std::string quota_key = request.get("X-ClickHouse-Quota", "");
    
    
    bool has_ssl_certificate_auth = (request.get("X-ClickHouse-SSL-Certificate-Auth", "") == "on");
    bool has_auth_headers = !user.empty() || !password.empty() || !quota_key.empty() || has_ssl_certificate_auth;