refactor: Streamline User identifiers (DEV-3155)

[BalduinLandolt]

Pull Request Checklist

Task Description/Number

Issue Number: DEV-

PR Type

• build/chore: maintenance tasks (no production code change)
• docs: documentation changes (no production code change)
• feat: represents new features
• fix: represents bug fixes
• perf: performance improvements
• refactor: represents production code refactoring
• test: adding or refactoring tests (no production code change)

Basic requirements for bug fixes and features

• Tests for the changes have been added
• Docs have been added / updated

Does this PR introduce a breaking change?

• Yes

Does this PR change client-test-data?

• Yes

[linear]

DEV-3155 Streamline User Identifier objects

[codecov]

Codecov Report

Attention: 124 lines in your changes are missing coverage. Please review.

Comparison is base (eac5751) 11.74% compared to head (37a426d) 11.76%.
Report is 6 commits behind head on main.

❗ Current head 37a426d differs from pull request most recent head d87283f. Consider uploading reports for the commit d87283f to get more accurate results

Files	Patch %	Lines
...g/knora/webapi/slice/admin/domain/model/User.scala	0.00%	40 Missing ⚠️
...ra/webapi/responders/admin/UsersResponderADM.scala	0.00%	28 Missing ⚠️
...knora/webapi/store/iiif/impl/SipiServiceLive.scala	0.00%	16 Missing ⚠️
...webapi/responders/admin/ProjectsResponderADM.scala	0.00%	11 Missing ⚠️
.../knora/webapi/slice/admin/api/UsersEndpoints.scala	0.00%	6 Missing ⚠️
...s/v2/responder/valuemessages/ValueMessagesV2.scala	58.33%	5 Missing ⚠️
...webapi/slice/admin/api/UsersEndpointsHandler.scala	0.00%	3 Missing ⚠️
...a/webapi/responders/admin/GroupsResponderADM.scala	0.00%	2 Missing ⚠️
...scala/org/knora/webapi/routing/Authenticator.scala	0.00%	2 Missing ⚠️
.../slice/admin/api/service/UsersADMRestService.scala	0.00%	2 Missing ⚠️
... and 9 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2979      +/-   ##
==========================================
+ Coverage   11.74%   11.76%   +0.02%     
==========================================
  Files         246      250       +4     
  Lines       22907    22934      +27     
==========================================
+ Hits         2690     2698       +8     
- Misses      20217    20236      +19     

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