What type of PR is this?

/kind enhancement

What does this PR do / why we need it:

Have you updated the necessary documentation?

• Documentation update is required by this PR.
• Documentation has been updated.

Which issue(s) this PR fixes:

Fixes #?

How to test changes / Special notes to the reviewer:

On a cluster install argo-rollouts -
kubectl create ns argo-rollouts
kubectl apply -n argo-rollouts -f https://github.com/argoproj/argo-rollouts/releases/latest/download/install.yaml
kubectl apply -n default -f https://github.com/argoproj/argo-rollouts/releases/latest/download/install.yaml

In the ArgoCD CR enable rollout extensions

spec: 
  server:
     enableRolloutsUI: true

Then in the argoCD UI, create an application

project: default
source:
  repoURL: https://github.com/saumeya/argocd-example-apps.git
  path: rollout-basic
  targetRevision: HEAD
destination:
  server: https://kubernetes.default.svc
  namespace: default

Git link - https://github.com/saumeya/argocd-example-apps.git
Path - rollout-basic

After the application is synced, you can click on the Rollout resource in the tree structure of the resources and see the Rollout Extension enabled in the UI -

To disable it, set the option the enableRolloutsUI: false . This restarts the server pod (takes a few seconds), also the UI browser needs to be reloaded, the plugin will disappear.

Note - Disabling the field in the CR is not removing the rollout extension - I have created an issue upstream and debugging it - argoproj-labs/rollout-extension#85