Implement dialog auth plugin

MariaDB and Percona Server use the [dialog](https://mariadb.com/kb/en/library/development-pluggable-authentication/#dialog-client-plugin) auth plugin instead of MySQL's `mysql_cleartext_password` plugin for PAM authentication.

This driver should come with built-in support, as PAM authentication is rather common. The implementation complexity seems low. MariaDB's client implementation can be found here: https://github.com/MariaDB/mariadb-connector-c/blob/master/plugins/auth/dialog.c