feat(argo-cd): Add ability to set Notification Services (#1220)

Signed-off-by: yu-croco <yuki.kita22@gmail.com>

charts/argo-cd/values.yaml

@@ -2580,3 +2580,9 @@ notifications:
 
       # -- [Node selector]
       nodeSelector: {}
+
+  # -- The notification services such as slack, email or custom webhook. This will be stored on `notifications.cm.name` ConfigMap.
+  ## For more information: https://argocd-notifications.readthedocs.io/en/stable/services/overview/
+  services: {}
+    # service.slack: |
+    #   token: $slack-token