…upgrade

- Extend `plugins:` pillar to accept dict entries `{name, url, force_cleanup?}`:
  downloads the .ez (filename derived from URL via basename) into the active
  /usr/lib/rabbitmq/lib/rabbitmq_server-<VER>/plugins/ dir BEFORE the broker
  restart, so /etc/rabbitmq/enabled_plugins references all stay resolvable on
  start-up. String entries (built-in plugins) keep their original behaviour.
- Cleanup of stale copies runs in both the active versioned dir and
  /usr/lib/rabbitmq/plugins/. Default pattern `<name>-<digit>*` keeps unrelated
  files (`<name>.bak`, `<name>-foo`) intact; per-plugin `force_cleanup: True`
  widens it to `<name>-*`.
- Add optional `rabbitmq:enable_all_feature_flag: True` which runs
  `rabbitmqctl enable_feature_flag all` BEFORE `rabbit_pkg`, gated by an
  `onlyif: rabbitmqctl status` so fresh installs skip it silently.