Allow to custom prepare of search term for LDAP

This change is related to issue #8760 and it seems to me a best way to meet general and specific interests.

Scenario:

A customer requires that search term in LDAP uses '*' . $term . '*'. Nextcloud in user_ldap/lib/Access.php uses $term . '*'. I agree that demand of only one is not enough to change a general behavior. But I think that must be possible to change behavior according to requirements.

At same time, that customer requires authentication with security token. We are using a customized version of user_saml (general changes were submitted here). For a better login performance, we have to restore search term to current Nextcloud implementation.

Then, I thought that issue #8760 can be closed if it is possible to replace search term expression on demand. It can be made though a customization app like Initialcheck. So, each sysadmin can configure LDAP search behaviour according his requirements without to change standard way of Nextcloud.