Implement method to get group's child groups using Keycloak 23 API

[tduong2049]

This doesn't resolve the issue #456 with GetGroups() and GetGroup() not including the child/sub groups in the results - because of changes in Keycloak 23's API:

• https://www.keycloak.org/docs/latest/release_notes/index.html#group-scalability
• GET /admin/realms/{realm}/groups Endpoint Not Returning subGroups, clientRoles, realmRoles Despite populateHierarchy and briefRepresentation settings" keycloak/keycloak#25053 (comment)

Rather, this is a separate method to get a group's child groups using the new GET /admin/realms/{realm}/groups/{id}/children endpoint in Keycloak 23's API - as mentioned in #456 (comment)

Method was tested locally with quay.io/keycloak/keycloak:23.0.4 image.

As mentioned in #456 (comment), this method could be expanded with a params GetChildGroupsParams parameter.

[Nerzal]

I guess we need to update the pipeline in order to test Keycloak 23+ Features.

[tduong2049]

Closing this PR to support #484