Allow to use a custom LDAP attribute as group name

[remyj38]

Allow to use a custom attribute as group name when using Group Search as Group membership strategy.
It's useful when cn isn't user-friendly and another attribute has the user-friendly value

Testing done

Added automated test using this feature.
Currently used on some Jenkins servers in our company.

### Submitter checklist
- [X] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [X] Ensure that the pull request title represents the desired changelog entry
- [X] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [X] Ensure you have provided tests - that demonstrates feature works or fixes the issue