feat: deploy argocd-image-updater via Helm

Deploys ArgoCD Image Updater into the argocd-image-updater namespace.
Vault-managed secrets provide registry credentials for git.unkin.net
and an ArgoCD API token.

Prerequisites before syncing:
- Create Vault role argocd-image-updater in k8s/au/syd1
- Populate kv/service/argocd-image-updater/registry-creds (key: creds, value: <user>:<token>)
- Create ArgoCD local user image-updater and store token at kv/service/argocd-image-updater/argocd-token

apps/base/argocd-image-updater/kustomization.yaml

@@ -0,0 +1,8 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml
+  - vaultauth.yaml
+  - vaultstaticsecret.yaml