[OAN-HELM-3] Helm chart for Adapter Service

[manjudr]

Description
Write the Helm chart for the Adapter Service, built on the oan-common
library chart (#53) for standard labels, probes, resources, service
account and ExternalSecret.

Acceptance criteria

• Chart consumes oan-common (labels/annotations, liveness+readiness
  probes, resources block, service account, ESO ExternalSecret)
• Per-environment values files
• Config and secrets read from the secrets store, never baked into the image
• helm lint passes; wired into CI