These changes help to support custom/contrib modules that want
to provide their own role migrations.
- Use skip_map_on_empty process plugin to ensure that skipped rows are not added to the map tables.
- Set a "role" pseudo-field, which is used by role ID matching.
- Move role ID matching process to farm_migrate_role migration group shared config.
- Move farm_migrate_role migration to optional config, dependent on farm_role_roles module.