Make it easier to create user with initial roles via REST API

Currently it is quite cumbersome to create users with an initial set of
realm or client roles since one needs to send multiple requests.

1) Request for creating the user
2) Request for setting the realms roles
3) Request for setting the client roles

It would be helpful if org.keycloak.services.resources.admin.UsersResource#createUser
would honor the provided role information in UserRepresentation by inspecting
the UserRepresentation#realmRoles and UserRepresentation#clientRoles.

For symmetry reasons it would also be helpful if org.keycloak.services.resources.admin.UsersResource#getUser
would also return this role mapping information.