add imagePullSecrets property

[icrc-fdeniger]

In some organization, using external docker registry like public.ecr.aws is not allowed and a proxy repository like Nexus must be used
This proxy can require credentials.

To be able to use this proxy the property imagePullSecrets should be added to the spec and configurable in values.yml

For instance it's done for official couchdb deployment:

https://github.com/apache/couchdb-helm/blob/a1dfd8b8a7e365ae1820b7ee1d57a05ca0fd18c5/couchdb/templates/statefulset.yaml#L40