Add auth and state access for terraform-rancher (#86)

## Why

The new `terraform-rancher` repo (manages Rancher's Authentik OIDC auth via the rancher2 provider) needs Vault auth + Consul state, mirroring the terraform-authentik runner (#78/#81/#82).

## Change

- `AppRole/terraform_rancher` + k8s auth role `woodpecker_terraform_rancher` (SA terraform-rancher in the woodpecker ns).
- Consul secret-backend role + ACL policy (`resources/secret_backend/consul_root/au/syd1/terraform-rancher.hcl`) 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.

Scoped the OAuth read to the `cattle-system` path specifically (rather than the `+` wildcard the authentik policy uses) since the Rancher runner only needs its own app's secret.

## Validation

pre-commit (terragrunt-hcl-fmt + yamllint) passed. CI plan will confirm.

Reviewed-on: #86
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/auth_approle_role/approle/terraform_rancher.yaml

@@ -0,0 +1,9 @@
+token_ttl: 120
+token_max_ttl: 120
+bind_secret_id: false
+token_bound_cidrs:
+  - "10.10.12.200/32"
+  - "198.18.25.102/32"
+  - "198.18.26.91/32"
+  - "198.18.27.40/32"
+use_deterministic_role_id: true

config/auth_kubernetes_role/k8s/au/syd1/woodpecker_terraform_rancher.yaml

@@ -0,0 +1,7 @@
+bound_service_account_names:
+  - terraform-rancher
+bound_service_account_namespaces:
+  - woodpecker
+token_ttl: 600
+token_max_ttl: 600
+audience: https://kubernetes.default.svc.cluster.local

config/consul_secret_backend_role/consul_root/au/syd1/terraform-rancher.yaml

@@ -0,0 +1,5 @@
+consul_roles:
+  - terraform-rancher
+ttl: 120
+max_ttl: 300
+datacenters: []

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

policies/kv/service/terraform/rancher.yaml

@@ -0,0 +1,18 @@
+# Allow the Terraform Rancher runner to read:
+#  - its own Rancher admin API token (rancher2 provider auth), and
+#  - the OAuth2/OIDC client secret backing the Rancher keycloakoidc AuthConfig
+#    (same secret Authentik sets on the provider).
+---
+rules:
+  - path: "kv/data/service/terraform/rancher"
+    capabilities:
+      - read
+  - path: "kv/data/kubernetes/namespace/cattle-system/default/oauth-credentials"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_rancher
+  k8s/au/syd1:
+    - woodpecker_terraform_rancher

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"
+}