[FEATURE] Add support for sub-groups

[Skyppid]

✨ Feature Description

Currently the API client does not offer any way to retrieve subgroups of a group. By default KC does not return subgroups (see https://forum.keycloak.org/t/api-is-not-showing-groups-and-sub-groups-info-after-upgraded-to-version-26/28391). For that there's a specific endpoint which seems to be not implemented yet. Alternatively maybe it's required to specify specific query parameters which are not passed through either.

🎯 Use Case

For a migration tool we need to retrieve subgroups, as those represent "organizations" we need to migrate into our new system.

💡 Proposed Solution

Implement endpoint https://www.keycloak.org/docs-api/latest/rest-api/index.html#_get_adminrealmsrealmgroupsgroup_idchildren

🔄 Alternative Solutions

NONE

📝 Additional Context

✅ Implementation Checklist

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

🔗 Related Issues/PRs