Allow Volumes/VolumeMounts to pass through to the Deployment

[aboukhal]

When trying to apply custom styling to ArgoCD, for example to color the banners as an easy distinction on which Cluster one is currently on, there is a documentation to be found here:
https://argo-cd.readthedocs.io/en/stable/operator-manual/custom-styles/

The documentation contains two alternatives on how to achieve that goal:

1. Manually change the argocd-server-Deployment to mount the additional CSS-File to the file system and use it from there.
2. Configure an external URL for the CSS file, spin up a web server, service and route just for a single file.

Alternative 1 does not work with the operator because the Deployment gets overwritten by it immediately.
Alternative 2 is not a sensible solution unless there is already some central storage for look-and-feels throughout a company. I'm not sure if this actually exists anywhere.

Describe the solution you'd like
There should be an option to declare Volumes and VolumeMounts directly in the ArgoCD resource. They should be automatically propagated to the argocd-server Deployment. Since the Deployment only contains one container, it could even be simplified further to a single object in the CRD, that contains all options for Deployment.spec.template.spec.volumes plus the field mountPath. In a first step, ConfigMap and Secret support might even cover the vast majority of use cases.

Describe alternatives you've considered
Alternatively, the Operator should not consider added Volumes or VolumeMounts a difference and should not overwrite them, even if something else in the ArgoCD resource has changed that necessitates a change to the Deployment. I believe this solution is worse than the one I suggested but it would still be much better than the Alternative 2 above.