[FEATURE] Auto discovery of credentials using image pull secrets for private repos

**Is your feature request related to a problem? Please describe.**
A cluster may contain pods with images from a large number of private repos (e.g. multi-tenant clusters), where configuring version-checker for each private repo can be quite tedious/possibly a never ending activity.

**Describe the solution you'd like**
Given that pods running in a cluster that are sourced from private repos already have credentials provided, it should be possible to automatically discover the credentials using the pods image pull secrets.

**Describe alternatives you've considered**
Nothing really viable I can think of.

**Additional context**
Given that this requires access to Kubernetes secrets, it should be toggleable on/off in the chart as not everyone may want to grant read access to secrets (or need to). If this feature is accepted, it may also be worth considering supporting OpenShifts [global image pull secret
](https://docs.openshift.com/container-platform/4.16/openshift_images/managing_images/using-image-pull-secrets.html#images-update-global-pull-secret_using-image-pull-secrets) so that any registries configured there can be authenticated to. Again the idea is to make it automatic, so that credentials would not need to be copied to version-checkers helm values file for every private registry/repo.

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**Message from the maintainers**:

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