Allow skipping user_external for specific users

[golanv]

It seems that Nextcloud does not provide any ability to prioritize various backend authentications. In my case, when enabling both user_ldap and user_external, user_external is always preferred with no real way to change this.

One possible approach is for user_external to skip authenticating users that are specified in some list (perhaps a list placed in config.php?). This could be easily tested in the various "checkPassword" functions for different backends (apps/user_external/lib/SSH.php, etc...):

public function checkPassword($uid, $password) {
                if (in_array($uid, ['user1'])) {
                        return false; // Skip user_external for these users
                }
...

Perhaps such a feature would be useful for other reasons as well?