kubectl-ate: filter get workers by namespace, assigned actor's atespace and selector

[Haven Xia (HavenXia)]

Should have sent out when top worker was added, thanks Luiz Oliveira (@laoj2) for remindering!

This allow kubectl ate get workers to filter workers by -n/--namespace, -a/--atespace or -l/--selector.

$ kubectl ate get workers
NAMESPACE          POOL      POD                              STATUS     ASSIGNED ACTOR
ate-demo-counter   counter   counter-worker-pool-7b9f8-x123   ASSIGNED   ate-demo-counter/counter/team-a/my-counter-1
ate-demo-counter   counter   counter-worker-pool-7b9f8-y456   FREE       <none>
ate-demo-glutton   glutton   glutton-worker-pool-5c2d1-a789   ASSIGNED   ate-demo-glutton/glutton/team-b/load-gen-1

$ kubectl ate get workers -a team-a
NAMESPACE          POOL      POD                              STATUS     ASSIGNED ACTOR
ate-demo-counter   counter   counter-worker-pool-7b9f8-x123   ASSIGNED   ate-demo-counter/counter/team-a/my-counter-1

# Scope to one WorkerPool namespace / filter by pool labels, kubectl-style
$ kubectl ate get workers -n ate-demo-counter
$ kubectl ate get workers -l ate.dev/worker-pool=counter

Fixes #585

• Tests pass
• Appropriate changes to documentation are included in the PR

[Luiz Oliveira (laoj2)]

Thanks for the quick fix!