Support gitops-engine.argoproj.io annotations

[alexmt]

The synchronization process is controlled by the following annotations:

• argocd.argoproj.io/sync-wave
• argocd.argoproj.io/hook
• argocd.argoproj.io/hook-delete-policy
• argocd.argoproj.io/sync-wave

We should get rid of argocd part since the engine is not specific to Argo CD. There are two possibilities:

• Make annotation "domain" customizable
• Support gitops-engine.argoproj.io/<type> in addition to argocd.argoproj.io/<type> .

[jaypipes]

I see no reason not to support the original annotations, actually. Those annotations refer to the ArgoCD user experience and contract and as such should stay the same. Long-term, it would be great to add support for any FluxCD annotations in addition to the ArgoCD ones. Thoughts?