[FEATURE] Add support for organization groups

[Tungsten78]

✨ Feature Description

Organization Groups were added in Keycloak 26.6.2. Would be great to extend library support for these new endpoints.

https://www.keycloak.org/2026/04/org-groups
https://www.keycloak.org/docs-api/latest/rest-api/index.html#_get_adminrealmsrealmorganizationsorg_idgroups

🎯 Use Case

Gives library access for Organization Groups

💡 Proposed Solution

Follow patterns established in recent addition of root organization support

📝 Additional Context

N/A

✅ Implementation Checklist

• Feature design document
• Unit tests
• Documentation updates
• API documentation (if applicable)
• Example code (if applicable)

🔗 Related Issues/PRs

#19