Implement support for admin listing of issued credentials

Admins are users with the credential-offer-create role. They create offers for others. They should also be able to list credentials issued to other users in the same realm.

Holders always receive their own credentials. Admins list another holder with `target_user`. Without `target_user`, admins still receive only their own credentials.

Context: https://github.com/ADORSYS-GIS/keycloak-oid4vc-mock-fe/issues/29 (point 2)
Parent: https://github.com/ADORSYS-GIS/token-status-link/issues/119

Review the existing credential-listing entry point and authorization checks, then trace target_user and the credential-offer-create role across same-realm handling. Verify that holders still see only their credentials, admins see another user’s credentials only with target_user, and admins without it see their own. Use the linked context and parent issues to clarify integration details.

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