Add PullSecret string field for the Image instead of PullSecretRef

[burmanm]

What this PR does: Adds "PullSecret" to the Image struct that has precedence over PullSecretRef. This is to ensure the compatibility with the cass-operator Image type (and to allow easier migration in the future) and adheres to the API documentation in Kubernetes which states for LocalObjectReference:

"// New uses of this type are discouraged because of difficulty describing its usage when embedded in APIs."

Also, deprecate PullSecretRef so we can in the future migrate away from it.

Which issue(s) this PR fixes: Fixes #

Checklist

• [ ] Changes manually tested
• [x] Automated Tests added/updated
• [ ] Documentation added/updated
• [ ] CHANGELOG.md updated (not required for documentation PRs)
• [x] CLA Signed: DataStax CLA

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Quality Gate failed

Failed conditions
6.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud