Wire ArgoCD to Authentik OIDC

ArgoCD had no external ingress and only local admin auth. This exposes
argocd-server behind the traefik-internal gateway and enables Authentik SSO,
so operators log in with their identity and group membership.

- argocd-cm: set url and oidc.config (Authentik issuer, argocd client,
  openid/profile/email scopes); client secret resolved from the argocd-oidc
  Secret via $argocd-oidc:client_secret
- argocd-rbac-cm: match on the groups claim; default role:readonly; map the
  argocd-admins Authentik group to role:admin
- argocd-cmd-params-cm: server.insecure=true so argocd-server serves HTTP
  behind the TLS-terminating gateway
- Add Gateway + HTTPRoutes for argocd.k8s.syd1.au.unkin.net (mirrors grafana)
- Add VaultAuth + VaultStaticSecret sourcing the OIDC client secret from
  kv/kubernetes/namespace/argocd/default/oauth-credentials into the
  argocd-oidc Secret (labelled part-of=argocd)

Note: the OIDC client secret must be seeded in Vault out of band, and
argocd-server needs a one-time rollout restart to pick up server.insecure.

clusters/au-syd1/bootstrap/argocd-oidc-vaultstaticsecret.yaml

@@ -0,0 +1,23 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: argocd-oidc
+  namespace: argocd
+spec:
+  vaultAuthRef: default
+  mount: kv
+  type: kv-v2
+  # Seeded out of band; same secret is read by terraform-authentik to configure
+  # the provider's client_secret. Key: client_secret.
+  path: kubernetes/namespace/argocd/default/oauth-credentials
+  refreshAfter: 5m
+  hmacSecretData: true
+  destination:
+    name: argocd-oidc
+    create: true
+    overwrite: true
+    # ArgoCD only resolves `$secret:key` references against Secrets carrying
+    # this label.
+    labels:
+      app.kubernetes.io/part-of: argocd