Add labelSelector option to filter the ArgoCD instances for reconciliation

[raghavi101]

What type of PR is this?

/kind enhancement

What does this PR do / why we need it:
This PR introduces an env variable and a flag : ARGOCD_LABEL_SELECTOR and --label-selector which can be used to assign distinct labels to the ArgoCD instances, enabling each controller instance to be tailored to oversee only the corresponding ArgoCD instances identified by the label selector. By implementing this approach, the desired outcome can be effectively attained through targeted management and configuration of the instances.

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:

1. Start by exporting a desired label-selector : export ARGOCD_LABEL_SELECTOR=foo=bar
2. On the terminal run the following to start the operator locally : make install run
3. Apply a basic argocd instance manifest. eg.

apiVersion: argoproj.io/v1beta1
kind: ArgoCD
metadata:
  name: test1
  namespace: test-argocd
  labels:
    example: basic
spec: {}

4. The above argocd instance will not be reconciled as it is not matched with the label selector option of the operator. To reconcile the argocd instance add the same label to it : kubectl label argocd test1 foo=bar -n test-argocd
5. The same applies to changes in the argocd instance which has been labelled and reconciled initially but if it gets unlabelled later kubectl label argocd test1 foo- -n test-argocd then it will not be reconciled by the operator.

[raghavi101]

Hi @ishitasequeira thanks for your review! I've updated the code with recent changes PTAL.

[ishitasequeira]

Left a couple of comments!!

[saumeya]

@raghavi101 Can you add steps/instructions to review the changes

[anandf]

Thanks @raghavi101 for these changes. Added few comments and suggestions. Please address them.

[ishitasequeira]

@raghavi101 , left some comments. PTAL.