Allow to custom prepare of search term for LDAP

[fgsl]

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.

[nextcloud-bot]

GitMate.io thinks possibly related issues are #9898 (Appstore: allow search across all categories), #3854 ("Resharing is not allowed" for LDAP users), #7175 (Allow to search for email address in user management), #6799 (search/results.css not overriding from custom theme), and #7180 (Search field).

[szaimen]

@nextcloud/ldap is this feasible?

[blizzz]

there was already one community attempt (or two) to introduce this, but they did not go into a mergable state. Thus, I'll still consider a PR if it is provided.