CustomResource lifecycle notifications for EventSources

[snowdrop-bot]

Currently the event sources are already aware when a custom resource is delete, we have a callback which is currently called for example for TimerEventSource, to cleanup all the timed events for the resource if it's not exists anymore.

Similar situation could happen for polling event source: operator-framework#651
Where in the variant where we would poll separately for each custom resource the event source should be aware of the lifecycle events of a custom resource. Mainly if a custom resource is created and deleted. So the used don't have to pass the custom resource all the time to the polling event source what he wants to poll.

So a feature where an event source could implement CustomResourceEventAware interface where it could receive events for the custom resource like if created,deleted and updated arrived.

So for example when a new custom resource appeared the polling custom resource could poll from that point.

see also: operator-framework#666

(This is an additive API change not necessarily needs to be in v2 first release)

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operator-framework#665

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$upstream:665$