feat(plugin-barman-cloud): add possibility to unset runAsUser and runAsGroup

[Pidu2]

This PR addresses the requirements of #673.

In an OpenShift environment, you usually do not want to set runAsUser or runAsGroup to something specific, instead it is chosen automatically by the cluster to a random value.

With this change, runAsUser and runAsGroup can be set to null and are therefore skipped when rendering.

---

values.yaml

containerSecurityContext:
  runAsUser: null
  runAsGroup: null

result

❯ helm template . --values values-barman.yaml -s templates/deployment.yaml | yq .spec.template.spec.containers.0.securityContext
allowPrivilegeEscalation: false
capabilities:
  drop:
    - ALL
readOnlyRootFilesystem: true
seccompProfile:
  type: RuntimeDefault

[phisco]

@Pidu2 can you fix the DCO failure in CI? otherwise LGTM 🙏