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.

resources/secret_backend/consul_root/au/syd1/terraform-rancher.hcl

@@ -0,0 +1,7 @@
+key_prefix "infra/terraform/rancher/" {
+  policy = "write"
+}
+
+session_prefix "" {
+  policy = "write"
+}