Add build timestamp to clusterstack status

[mgibson1121]

As a user of kp I want to know when the last time a particular stack was updated with a new build and run image pair So that I can make an informed decision around when to next update the stack

Feedback from kpack user interviews with VMware support engineers. Seems to me the easiest way to address this would be to surface time stamps in the output of kp clusterstack status like we do on kp image status. There is a risk of redundant work, seems to me that the main purpose of updating a stack is to trigger Image rebases. And we already provide those timestamps on the image status. Seems to be very small effort though, if we implemented it I doubt we would ever regret doing so.