ApplicationSet: Allow preserving state of custom annotations

[lacarvalho91]

Summary

An ApplicationSet should be able to be configured to allow preserving the state of custom annotations. This is already hard-coded for the Argo CD Notifications and the Argo CD refresh type annotations, the reason this was done for those can also apply to custom annotations.

Motivation

I'm using a custom plugin that patches the Application being refreshed with some context that is only available at refresh time. I need that context to be available to Argo CD Notifications (and other external systems) so I am storing that context in a custom annotation. The problem now is that the ApplicationSet controller will remove the annotation as it isn't defined on the ApplicationSet. Its a common pattern for operators and such to use custom annotations to keep some state, as seen by the annotations Argo CD itself uses.

Proposal

I propose adding a new field to the ApplicationSet that allows users to configure annotations that should be preserved. This would look something like: preservedFields.annotations: ["custom-anno"]. This structure can be evolved in the future to support preserving other fields (labels could be a good candidate), though you may want to avoid allowing arbitrary fields to be preserved.