Add Build watch support

this needs to be updated at the same time that makeBuildListKey is updated.

Depending on the order of your pull request, makeBuildListKey needs to just return "/registry/builds/" otherwise, this will result in a list key that is namespace scoped (or would if there wasnt a bug in the version of k8s that is here).

see: https://github.com/GoogleCloudPlatform/kubernetes/pull/2074