Mark job pods not to use Istio's envoy sidecar (#18415)

* Mark job pods not to use Istio's envoy sidecar

Istio injects sidecars into pods to implement mTLS between pods. Jobs
usually don't know about this, so they don't signal the Envoy process
to stop when the job finishes. Since at least one process is running
in the pod, Kubernetes doesn't consider the job to be completed, so it
lingers.

By adding the `sidecar.istio.io/inject` annotation set to `"false"`,
we let Istio know that it should not inject the sidecar. If Istio is
not installed, then this has no impact.

* Support arbitrary job annotations in the Helm chart

Rather than focus on Istio, this allows arbitrary annotations for job pods.

* Add in-line documentation for pod/job annotations