Add ability to set MYSQL_CLIENT_FLAGS via environment variables

[TobiTh]

It is currently not possible to set the MYSQL_CLIENT_FLAGS via the env vars.
The only known possible workaround to me is currently to not set the env vars and mount a modified wp-config.php file with the flags set. But this breaks the existing functionality in the docker image.

I need this setting to enforce SSL via:
define('MYSQL_CLIENT_FLAGS', MYSQLI_CLIENT_SSL);

[tianon]

With #557, the supported means of setting additional flags like this is WORDPRESS_CONFIG_EXTRA (in which you'd put that entire define line).