Add settings to control observer behavior

[desertaxle]

Add a KubernetesObserverSettings object to allow configuration of the namespaces the observer watches and additional labels to filter on.

The namespaces the observer watches can be set with the PREFECT_INTEGRATIONS_KUBERNETES_OBSERVER_NAMESPACES environment variable like this:

export PREFECT_INTEGRATIONS_KUBERNETES_OBSERVER_NAMESPACES="default,another-one"

and additional labels to filter on can be set like this:

export PREFECT_INTEGRATIONS_KUBERNETES_OBSERVER_ADDITIONAL_LABEL_FILTERS="prefect.io/deployment-id=not-a-real-deployment-id,prefect.io/deployment-id=also-not-a-real-deployment-id"

Both these settings can be used to assign observers to specific domains if multiple observers are running in the same cluster.

[desertaxle]

@meggers I think these changes will make it easier to test the new version of the Kubernetes worker in a smaller scope. Let me know if there's anything else you think would be helpful!

[meggers]

@meggers I think these changes will make it easier to test the new version of the Kubernetes worker in a smaller scope. Let me know if there's anything else you think would be helpful!

Thanks @desertaxle, it looks like this will allow us to use specific namespaces and filter to flows more specific to our worker with prefect.io/work-pool-id. I think that should help us! Appreciate it.

[zzstoatzz]

looks sweet!