Change the return type of GetUserGroups to Group

[azerbe]

Is your feature request related to a problem? Please describe.

When you query for UserGroups (GET /{realm}/users/{id}/groups) you get a Grouprepresentation as result.
But currently the return type is a UserGroup. This doesn't allow the use of querys like

briefRep := false
groups, err := client.GetUserGroups(ctx, k.token.AccessToken, k.Realm, id, gocloak.GetGroupsParams{BriefRepresentation: &briefRep})

Something that is totally possible, if you query the API directly and if you are interested in informations like group attributes.

Describe the solution you'd like
Return []Group instead.

[SVilgelm]

So, my question: Is it the bug we have from the very beginning or the return type is changed in some version?
Looks like that the USerGroup is a subset of Group, so I don't see any problems, it should be backward compatible with Keycloak's API, but form the Gocloak's point of view we must to release new major version, since it breaks backward compatibility