Allow opni-agent Secret agent-config to be created outside of Helm

[ron1]

A common GitOps practice is to create and manage Secrets outside of package managers like Helm and Kustomize. The opni-agent helm values file currently does not support this practice since it always creates the agent-config Secret with inline sensitive bootstrap fields including the token and pin. Consider enhancing the opni-agent chart to allow this Secret to be created external to Helm.