Add the ability to define imagePullSecrets for service account

[cognifloyd]

If pods do not have imagePullSecrets, they get populated from the ServiceAccount[1]. In our k8s cluster, we prefer defining them on the ServiceAccount so that there are fewer places to update if a change is required.

Effectively, this means we can use serviceAccount.pullSecret instead of image.pullSecret in values.yaml.

[1] From the k8s docs:

5. If the pod does not contain any imagePullSecrets, then imagePullSecrets of the ServiceAccount are added to the pod.

TODO:

• PR description
• changelog entry

[arm4b]

Very helpful, thanks for the contribution! 👍