Added Validations for WorkloadGroups, fixed issue with references.

[hhovsepy]

Describe the change

Added 2 new Validation Warning messages for Workload Groups:
"Warning KIA1701 Service Account not found in this namespace".
"Warning KIA1702 More than one Workload Group with duplicate labels found in the same namespace"
Kiali does not list ServiceAccounts directly from the system but is listing them from the Pods.
That is why the notification severity is Warning not a Error.

Steps to test the PR

Create a WorkloadGroup with non existing Service Account, or with Service Account from other namespace, and with labels which exist in another WG in the same namespace.

apiVersion: networking.istio.io/v1
kind: WorkloadGroup
metadata:
  name: ratings-vm
  namespace: bookinfo
spec:
  metadata:
    labels:
      app: ratings-vm
  template:
    serviceAccount: bookinfo-ratings
---
apiVersion: networking.istio.io/v1
kind: WorkloadGroup
metadata:
  name: ratings-vm2
  namespace: bookinfo
spec:
  metadata:
    labels:
      app: ratings-vm
  template:
    serviceAccount: wrong

Automation testing

Added unit tests.

Issue reference

#8058
Docs: kiali/kiali.io#853