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.

environments/au/syd1/terragrunt.hcl

@@ -71,6 +71,8 @@ inputs = {
   litellm_secret_backend         = local.config.litellm_secret_backend
   litellm_secret_backend_role    = local.config.litellm_secret_backend_role
   plugins                        = local.config.plugins
+  gpg_secret_backend             = local.config.gpg_secret_backend
+  gpg_key                        = local.config.gpg_key
 
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map