Reuse argocd repo server image with config management plugins

[jimmyjones2]

Is your feature request related to a problem? Please describe.
I'm porting a config management plugin from the old ConfigMap method. This used the existing repo server image as it had all the necessary tools - helm etc. However with the new method of creating config management plugins, I'll need to create or find an image with the necessary tools, and keep it updated.

Describe the solution you'd like
I'd like my plugin to be able to use the existing repo server image associated with the version of ArgoCD it's being deployed into. For example if spec.repo.sidecarContainers[].image is null, the repo server image is used. When the operator / ArgoCD is updated, the image will be updated.

Describe alternatives you've considered

• Hardcode the image to the current argocd repo server image - but won't get upgraded when ArgoCD is, and image address depends on if deployed in Openshift or in vanilla Kubernetes
• Build a new image from scratch - but then have to take responsibility of keeping it updated