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/vaultauth.yaml

@@ -0,0 +1,18 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultAuth
+metadata:
+  name: default
+  namespace: argocd-image-updater
+spec:
+  allowedNamespaces:
+    - argocd-image-updater
+  kubernetes:
+    audiences:
+      - vault
+    role: argocd-image-updater
+    serviceAccount: argocd-image-updater
+    tokenExpirationSeconds: 600
+  method: kubernetes
+  mount: k8s/au/syd1
+  vaultConnectionRef: vso-system/default