RFR: Support yaml metadata for actions and rules

* Add: Yaml support for actions and rules meta. Both yaml and yml extensions
are supported.
* Add: yaml, yml and json meta for sample action in fixtures for testing.
@manasdk Yeah, in any case, I think we should move towards using `extra`.

It's a documented way of doing it and we get additional context (dictionary key) which we don't get in this case since it's using positional args. We could use keyword args, but I like `extra` approach more.