Support for Status Notifications (orchestrator-side)

Currently, EMCO orchestrator supports status for the applications deployed by EMCO. The status API needs to be explicitly called by the user to get the status. Some users don't like to poll for the status on a periodic basis. For that, it is also needed for users to get notified whenever the status changes. A use case is a microservice, which is interacting with EMCO. A microservice can register for status notifications and when notified can get the detailed status via status API. Microservice can register for status notification by giving appContext and notification criteria (any change, change only for a set of resources across a set of clusters). EMCO components like the orchestrator, clm are expected to honor this registration and inform the target endpoints when it observes the status change.