charts/timescaledb-single: add imagePullSecrets

[ayeks]

Using a private repository was not possible before, so I added the imagePullSecrets to the StatefulSet.

You can now define an existing secret from the same namespace. Example:

image:
  repository: docker.yourcompany.com/timescale/timescaledb-ha
  tag: pg13.4-ts2.4.2-p0
  pullPolicy: IfNotPresent

imagePullSecrets:
- name: registry-credentials

Checkout https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry/ for detailed information.

[c-p-b]

Any chance this will make it in the next release? Currently I have to run a local version of this chart only because of this change.