Add caching_sha2_password Support in MaxScale

MaxScale currently does not support MariaDB users using the caching_sha2_password authentication plugin.

When a user is changed from mysql_native_password to caching_sha2_password, connections through MaxScale fail with:

ERROR 1045 (28000): Access denied

MaxScale logs show:

Authentication plugin error.

Last error: Error from 'server1': Authentication plugin error.

Steps to Reproduce

1. Create a MariaDB user using mysql_native_password.
2. Confirm that the user can connect through MaxScale.
3. Change the user to caching_sha2_password.
4. Reload the MaxScale service.
5. Try connecting through the MaxScale listener.
6. The connection fails with an authentication error.