feat: get child groups and get unregistered required actions

[surasithaof]

After I upgraded Keycloak from version 19 to 25.0.1, I found the issue when try to call GetGroup and GetGroupByPath that the response does not include the sub groups.

• I add the function to call to get child groups by groupID, and also fix the tests to be compatible with Keycloak version 25.0.1 as well.
• I already updated the error messages in test cases because of some error message has changed.
• And it I need to fix TestGocloak_CreateAndGetRequiredAction because it cannot call to register required action. So I added GetUnregisteredRequiredActions for this test refers to this test suit.