Add support for Label-based policies

[pacphi]

The cf v3 API has support for assigning metadata (e.g., labels and annotations) to apps and (possibly in the future) service instances.

An interesting use case from a policy enforcement perspective is to have developers be required to label apps in conjunction with cf push. The cf CLI nor the cf-java-client currently have support so we'd have to build out something perhaps with a Spring WebFlux.

If an operator could apply and search for labels on apps (not unlike applying and filtering on tagged compute resources in AWS, Azure, or GCP), then there is another lever to: a) manage finite compute capacity (what cf-butler is arguably good at), b) establish compliance. Case b) is super-interesting because it can take many forms (regulation, cost control).

So, what we want to do here is offer yet another way to define a policy. If an app does not have 1-n labels defined then go ahead and stop and delete the app (and optionally any bound services).

[pacphi]

Not implementing as described. Not going to support destructive action in this manner. Just focusing on reporting and notification. See Issue #386.