factoring watch/list functionality out of config map updater and deployment cache

[arschles]

The config map updater and deployment cache share similar core functionality. They:

• open a watch stream for a resource
• periodically ping the same resource
• update an internal storage system based on changes from the watch stream and updates from the periodic pinger
• shut down when a context is done

This factors out the functionality in the first two and last bullet points into a separate WatcherLister struct. This is similar to the informers functionality in k8s.io/client-go, but significantly simpler, and agnostic of any storage concerns. taking that strategy achieves two major things:

• keeps storage and event processing orthogonal to any watch/list/reconcile functionality
• prevents much of the loss of type safety generally associated with the informers libraries

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Tests for the WatcherLister
• Any necessary documentation is added, such as:
  • README.md
  • The docs/ directory
  • The docs repo