Allow to configure handlers (identified by path in URL) for prepared queries.

Both SELECT and INSERT queries should be supported.
SELECT query can be parametrized (see #5331).

Example:

Instead of http://localhost:8123/?query=INSERT%20INTO%20table%20...
the user might want to send data to http://localhost:8123/my_custom_handler/

This will allow more easy integration with third-party tools.