v4.2.0

Release v4.2.0

feat: add subscription/notification support and batch calls

- Add `ubus::subscription` RAII handle for subscribing to object
  notifications (`subscribe()` / `notify()`).
- Add `call_batch()` for executing multiple synchronous calls in one
  go, returning results in order.
- Add `subscribe_cb` to `add_object()` so objects know when they
  gain or lose subscribers.
- Add subscriber.cpp and notifier.cpp examples demonstrating the
  pub/sub pattern.
- Add examples for subscriber and notifier and move all examples to
  examples directory in root of project.
- Update README.md with full API documentation for the new features.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>