Define new event to summarize debugging status for container images

`skaffold debug` merely issues a warning to the console on encountering a container that cannot be configured for debugging. This warning is easily missed. Users find having a container not appear to be debuggable is puzzling.

Skaffold should issue an event to report when a container cannot be configured for debugging. Initial thoughts are that there should be a single event summarizing the debugging enablement for all images. As an image can appear in multiple workload resources, and maybe only some cannot be debugged, it could be easier to raise events for each such container reference encountered.