Add auth and state access for terraform-rancher

Mirrors the terraform-authentik runner setup (#78/#81/#82) for the new
terraform-rancher repo, which manages Rancher's Authentik OIDC auth via the
rancher2 provider.

- AppRole terraform_rancher + k8s auth role woodpecker_terraform_rancher.
- Consul secret backend role + ACL policy granting write to the
  infra/terraform/rancher/ state prefix.
- Vault policies: read the Rancher admin API token
  (kv/service/terraform/rancher) and the keycloakoidc client secret
  (kv/kubernetes/namespace/cattle-system/default/oauth-credentials), plus the
  consul_root state creds.

policies/consul_root/au/syd1/creds/terraform-rancher.yaml

@@ -0,0 +1,11 @@
+---
+rules:
+  - path: "consul_root/au/syd1/creds/terraform-rancher"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_rancher
+  k8s/au/syd1:
+    - woodpecker_terraform_rancher