Mount the gpg engine and manage a 'pass' key

Mount the gpg secrets engine (the plugin is registered separately via the
plugin module + config/plugins, #89) and manage OpenPGP keys with the
gpgvaultsecret provider.

- gpg_secret_backend module: mounts the registered plugin type; depends_on the
  plugin module so the mount waits for catalog registration.
- gpg_key module (via the gpgvaultsecret provider, registered in root.hcl):
  create/configure keys; wired through vault_cluster + config discovery
  (config/gpg_key/<backend>/<name>.yaml).
- config/gpg_secret_backend/gpg.yaml mounts at gpg/; config/gpg_key/gpg/pass.yaml
  is an rsa-4096 'pass' key for password-store (private key stays in Vault;
  clients decrypt via gpg/decrypt/pass).

Rebased onto master after the access (#88) and plugin-import (#89) PRs merged.

policies/rancher/admin.yaml

@@ -1,55 +0,0 @@
-# Allow the vault deployer to import the rancher plugin and manage its engine
-# (config, seeded service accounts, and roles).
-#
-# terraform-vault registers the plugin itself (vault_plugin -> sys/plugins/catalog,
-# a sudo-protected path) and manages the engine via the ranchervaultsecret
-# provider, so the deployer needs catalog access on top of the mount access it
-# already has (sys/mounts/*). Without this, apply 403s on the plugin
-# registration and on rancher/{config,service-accounts,roles} writes.
----
-rules:
-  # Import / register (and deregister) the rancher plugin in the catalog.
-  - path: "sys/plugins/catalog/secret/vault-plugin-secrets-rancher"
-    capabilities:
-      - create
-      - read
-      - update
-      - delete
-      - sudo
-  # Engine connection config.
-  - path: "rancher/config"
-    capabilities:
-      - create
-      - read
-      - update
-      - delete
-  # Seeded, auto-rotated service-account tokens (+ manual rotate).
-  - path: "rancher/service-accounts/*"
-    capabilities:
-      - create
-      - read
-      - update
-      - delete
-      - list
-  - path: "rancher/service-accounts"
-    capabilities:
-      - read
-      - list
-  # Token-minting roles.
-  - path: "rancher/roles/*"
-    capabilities:
-      - create
-      - read
-      - update
-      - delete
-      - list
-  - path: "rancher/roles"
-    capabilities:
-      - read
-      - list
-
-auth:
-  approle:
-    - tf_vault
-  k8s/au/syd1:
-    - woodpecker_terraform_vault