extend watchevent with bookmark type

[clux]

a 1.15ism. See https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/20190206-watch-bookmark.md

[clux]

This is now beta in 1.16 and better documented in https://kubernetes.io/docs/reference/using-api/api-concepts/#watch-bookmarks

Probably just extending the WatchEvent and adding in the flag to the query params. I'm guessing it's ignored if it's not supported, but it should probably be user controlled somehow. Maybe via the reflector / informer Params struct.