Add support for adding CSI volumes as Additional Volumes

[nilushancosta]

Description

This PR adds the capability to add CSI volumes as AdditionalVolumes in the OpenSearch cluster

Issues Resolved

Closes https://github.com/opensearch-project/opensearch-k8s-operator/issues/275

Check List

• [x] Commits are signed per the DCO using --signoff
• [x] Unittest added for the new/changed functionality and all unit tests are successful
• [x] Customer-visible features documented
• [x] No linter warnings (make lint)

If CRDs are changed:

• [x] CRD YAMLs updated (make manifests) and also copied into the helm chart
• [x] Changes to CRDs documented

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[nilushancosta]

Thanks for the feedback @swoehrl-mw . I completed the checklist items regarding CRD changes.