Add enhancement proposal for auto create Argo CD cluster secrets using an external cluster registry

[mikeshng]

What type of PR is this? /kind enhancement

What does this PR do / why we need it: Add an enhancement proposal to further the discussion on https://github.com/argoproj-labs/argocd-operator/issues/869 issue about automated Argo CD cluster secrets creation via external cluster registry.

Have you updated the necessary documentation?

• [not applicable] Documentation update is required by this PR.
• [not applicable] Documentation has been updated.

Which issue(s) this PR fixes:

https://github.com/argoproj-labs/argocd-operator/issues/869

How to test changes / Special notes to the reviewer: not applicable

[mikeshng]

CC @iam-veeramalla @xiangjingli @jnpacker @ch-stark