Support for K8S Probes

[csviri]

This issue is about supporting Kubernetes Probes from the framework.

The main question is how to map probes to operators in general:

• Liveness - is operator started? (So start method called?) Is there any other meaningful way?.
  Should be this bound to sync timeout: Event Source Sync Timeout #1318 ? So if out of sync it is basically not alive?

• Readyness (or Startup?) - probes is naturally can be bidden to event source sync, so the operator is ready when event sources are already synced.

The API:

Operator operator = new Operator()
...
operator.start()

// from this point the methods
boolean ready = operator.isReady()
boolean alive = operator.isAlive()

User later can call those methods from a servlet container or whatever integration chooses with K8S for the probes.

[csviri]

An other thing we should check in the probes if the informers are watching:
fabric8io/kubernetes-client#4340 (comment)

So probably the operator is not ready if the informers are not watching.

But this is related also with the issue: #1405
Where we want to start informers, and if there is not permission for the informer (and not watching) that is fine.
This however this probably requires explicit checking of permission.