add volumes support to ExecutorPlugin

[Dejan Golja]

Summary

I was looking at the executor plugins spec definition, which is basically subset of the containers definition. I was wondering if we should also extend the spec with the Volumes definition or is there an security issue with that? I am mostly after a way how I could mount secrets/config maps volumes inside the plugin(s).

Use Cases

apiVersion: argoproj.io/v1alpha1
kind: ExecutorPlugin
metadata:
  name: hello
spec:
  sidecar:
    volumes:
      - name: secret-volume
        secret:
          secretName: dotfile-secret
    container:
      ...
      volumeMounts:
      - name: secret-volume
        readOnly: true
        mountPath: "/etc/secret-volume"
      ...

I believe there is value in having access to secret volumes, which can be managed through ESO for specific use cases where you want to dynamically manage secrets for plugins.

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[Dejan Golja]

If no disagreement happy to provide the code implementation.