Improve health check of Istio subcomponents

Kiali assumes by default that Istio control plane is up and running.
If some of the subcomponents of Istio are not healthy (i.e. a pod is down), probably Istio won't work correctly.
It would be very useful if Kiali could add some support to those situations.

This epic is to research and track effort in detect health check of Istio subcomponents.