fix: [OCISDEV-868] search for username if group attribute is memberUid

[jvillafanez]

Description

Group membership assumed LDAP always used either "member" or "uniqueMember" attributes (whose values are DNs), so errors happens when "memberUid" is used.

owncloud/reva#719 might be required because it fixes a related bug.

Related Issue

• Fixes <issue_link>

Motivation and Context

memberUid can be used as a valid group membership attribute.

How Has This Been Tested?

Manually tested with posixGroups

Screenshots (if appropriate):

Side note: the web UI shows the "cn" or displayname of the user, not the "uid"

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

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