feat: OpenLDAP backend for user-info-fetcher

[dervoeti]

Description

Fixes #523

Decision about the CRD change has not been accepted yet.

One performance problem might be that get_user_info creates a new LDAP connection every time it's called. A connection pool might be a better solution but I did not want to overengineer it, especially since the AD backend does it the same way.

I also only added support for authentication via bind credentials, as I don't think we support auth via Kerberos in other places were we connect to LDAP. We could think about adding support for this if it's needed, I skipped it for now.

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