Add imagePullSecrets support to Helm chart

[animeshd9]

The Spin Operator Helm chart currently doesn't provide an option to configure imagePullSecrets for the Spin Operator Deployment.

This causes issues when the operator image is hosted in a private registry, as users need to manually patch the Deployment after installation.

Proposed change

Add support in values.yaml:

imagePullSecrets:
  - name: registry-secret

and propagate it to the operator Deployment:

spec:
  template:
    spec:
      imagePullSecrets:
        - name: registry-secret

This would allow users to configure private registry credentials directly through Helm.

[animeshd9]

Raised a PR #500