Add support to change the authentication plugin (MYSQL_DEFAULT_AUTH)

[macboy012]

A trivial change to allow for setting the authentication plugin used when connecting to mysql.
The auth_plugin parameter is the same parameter name used by mysql-connector-python and seemed sensible.

[methane]

Why do you want this option?
As far as I know, this option is totally useless.

[macboy012]

I'm using Proxysql which only supports mysql_native_password. When using libmysqclient21 the new default is caching_sha2_password so I'm unable to connect to Proxysql.
I understand there are some protocol capabilities for negotiating authentication method, but these seem to be unsupported by Proxysql. So, my solution is to set the correct auth plugin before connecting.