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.

bool has_http_credentials = request.hasCredentials();
    bool has_credentials_in_query_params = params.has("user") || params.has("password") || params.has("quota_key");
    std::string spnego_challenge;
    std::string certificate_common_name;
    if (has_auth_headers)
    {
        
        if (has_http_credentials)
            throw Exception("Invalid authentication: it is not allowed to use SSL certificate authentication and Authorization HTTP header simultaneously", ErrorCodes::AUTHENTICATION_FAILED);
        if (has_credentials_in_query_params)
            throw Exception("Invalid authentication: it is not allowed to use SSL certificate authentication and authentication via parameters simultaneously simultaneously", ErrorCodes::AUTHENTICATION_FAILED);
        if (has_ssl_certificate_auth)
        {
#if USE_SSL
            if (!password.empty())
                throw Exception("Invalid authentication: it is not allowed to use SSL certificate authentication and authentication via password simultaneously", ErrorCodes::AUTHENTICATION_FAILED);
            if (request.havePeerCertificate())
                certificate_common_name = request.peerCertificate().commonName();
            if (certificate_common_name.empty())
                throw Exception("Invalid authentication: SSL certificate authentication requires nonempty certificate's Common Name", ErrorCodes::AUTHENTICATION_FAILED);
#else
            throw Exception(
                "SSL certificate authentication disabled because ClickHouse was built without SSL library",
                ErrorCodes::SUPPORT_IS_DISABLED);
#endif
        }
    }
    else if (has_http_credentials)
    {