feat(chart): add existingSecret wiring

[AmmonsIO]

Summary

This adds a secrets.existingSecrets value that will override the creation of a secret in cluster without having to have external secrets operator. If a user is using encrypted secrets yamls or a secrets operator other than ESO they currently need to make and maintain modifications to the chart. This sets up the infrastructure for them to just reference an existing secret. It includes what the expected values in the secret should be. It also sets up a helper function for determining what the secret name that should be referenced by the deployments are.

Closes #158

Test Checklist

• Helm template renders without errors
• [] Terraform validates/plans cleanly
• [] Manually verified on staging cluster (if applicable)
• [] Rollback strategy documented

Infrastructure

• Helm: helm template renders correctly
• [] Terraform: terraform validate passes
• [] Terraform: terraform plan shows expected changes
• [] ArgoCD: Application manifests are valid
• [] N/A - documentation only

[github-actions]

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