Implement group_filter for LDAP integration

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/16230

In some LDAP systems users and groups appear within the same OU structure. In this case, when configuring a group_base and trying to link LDAP groups in the UI, both users and groups show in the autocomplete field. Supporting a group_filter option in configuration would allow administrators to set a filter such as (objectClass=group)

cc/ @JobV I don't think this would be too complicated. Please schedule if you think it sounds like a good idea.

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@DouweM This was scheduled for 8.8 but let's reschedule for a future release, please.

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mentioned in issue #666

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Same thing? https://gitlab.com/gitlab-org/gitlab-ee/issues/1040

Please, important feature!

mentioned in issue #1040

Requested by a customer in https://gitlab.zendesk.com/agent/tickets/71591

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@DouweM is it clear what needs to be done here?

cc @dblessing

@mydigitalself Yep, clear enough once someone understands LDAP fundamentals.

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We are now starting work on improving LDAP capabilities, apologies this has taken so long to get around to.

If you take a dive into this code, this might be solved in the same go easily: https://gitlab.com/gitlab-org/gitlab-ee/issues/1358.

Thank you for considering it.

assigned to @mydigitalself

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