cmd/k8s-operator: set imagePullSecrets on proxy ServiceAccounts

The Helm chart only applied imagePullSecrets to the operator Deployment's
pod spec, so proxy pods (which use a different ServiceAccount) never
received them and could not pull images from private registries.

Apply imagePullSecrets to the operator, proxies, and kube-apiserver-auth-proxy
ServiceAccounts.

Updates #16772

Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>