enhancement: Added oportuniy set ImagePullSecret

[eunames]

Summary

Added oportuniy set ImagePullSecret param to manifest of job. It allows use a private registry for images when we use $BACKUP_IMAGE.

Fixes https://github.com/k8up-io/k8up/issues/567

Checklist

For Code changes

• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] PR contains the label area:operator
• [x] Link this PR to related issues
• [ ] I have not made any changes in the charts/ directory.

For Helm Chart changes

• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] PR contains the label area:chart
• [x] PR contains the chart label, e.g. chart:k8up
• [x] Variables are documented in the values.yaml using the format required by Helm-Docs.
• [x] Chart Version bumped if immediate release after merging is planned
• [x] I have run make chart-docs
• [x] Link this PR to related code release or other issues.

[TheBigLee]

@eunames Could you rebase your branch so we can have a look at the change again and merge it if everything looks good?