Use watch for Kubernetes strategy

Thanks for creating an amazing library that makes clustering so much simpler!

Kubernetes has the ability to `watch` a given resource. When it is watched, the transfer encoding is set to `chunked`, and any updates after the specified version will be sent down in new chunks. This way the node wouldn't have to poll the endpoint every 5 seconds, but could create a long-lived connection to it and then just update whenever new chunks are received. I believe this would allow for node list updates to occur much closer to real-time.

Are there any drawbacks to supporting `watch` requests?

Would there be support for a PR implementing this?

If yes, would it make more sense to update the existing `Cluster.Strategy.Kubernetes` module or to create a new one like `Cluster.Strategy.KubernetesWatched` or something like that?

It seems like the high-level functionality will be exactly the same but more performant. That being said, I don't know when the `watch` capability was added to Kubernetes, so there is potential that it would be a breaking change for those who might be on older version. The docs reference watch in version 1.14, but I haven't looked further back than that.