Implement Helm release handler for Helm storage backend

[pkosiec]

Description

Implement Helm release storage backend:

• Extend Helm backend under /cmd/helm-storage-backend. Implement gRPC server. Types are already generated.
  • Support all possible releases backends (ConfigMap/Secret/etc). Maybe we will need to extend the backend.context to allow specifying it.
  • Support only GetValue method, the rest just validate data (e.g. onDelete checks whether a given Helm release exist).
    • To consider what exactly we want to validate - Should we really return error if a given Helm release doesn't exist in OnDelete method? It could be deleted externally. Or perhaps we just need to validate that in onCreate, onUpdate, and locking/unlocking? 🤔
• Helm backend Type manifests is already defined. Ensure that it is up-to-date at the end of the task.
• Keep the same shape of data: https://github.com/capactio/hub-manifests/blob/main/manifests/type/helm/chart/release.yaml

Related issues

See epic #650