Add a plugin-import module + config/plugins for catalog registration (#89)

Split plugin catalog registration out of the per-engine backend modules into its own concern (previously bundled into #87's gpg_secret_backend).

## Changes
- New generic `plugin` module (`vault_plugin`: type/name/command/sha256/plugin_version) that imports a binary into the catalog.
- New `config/plugins/` discovery group (filename = catalog name = mount type), wired through `vault_cluster` (`plugins` variable + module) and the syd1 environment.
- `config/plugins/vault-plugin-secrets-gpg.yaml` pins the released v0.1.0 binary sha256 (`0e92d740…a7b20`, from the published RPM). Puppet installs the RPM floating, so bump this in lockstep on upgrade.

Any engine now registers its plugin by dropping a file in `config/plugins/`; its `*_secret_backend` module just mounts the registered type.

Needs the deployer's plugin-catalog access (#88). Merge order: **#88 → this → #87**.

Reviewed-on: #89
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/config.hcl

@@ -198,5 +198,12 @@ locals {
       })
       if startswith(file_path, "litellm_secret_backend_role/")
     }
+    plugins = {
+      for file_path, content in local.all_configs :
+      trimsuffix(basename(file_path), ".yaml") => merge(content, {
+        name = trimsuffix(basename(file_path), ".yaml")
+      })
+      if startswith(file_path, "plugins/")
+    }
   }
 }

config/plugins/vault-plugin-secrets-gpg.yaml

@@ -0,0 +1,10 @@
+# config/plugins/vault-plugin-secrets-gpg.yaml
+# Imports (registers) the gpg secrets plugin in the catalog. Filename = catalog
+# name = mount type. The binary is installed on the OpenBao nodes by Puppet
+# (openbao-plugin-secrets-gpg RPM -> /opt/openbao-plugins/vault-plugin-secrets-gpg).
+#
+# sha256 pins the released v0.1.0 binary; bump it in lockstep with any RPM
+# upgrade or OpenBao will refuse to launch the plugin.
+type: secret
+command: vault-plugin-secrets-gpg
+sha256: "0e92d7408795688badb55789bc1604e8f1dd4d71998656c7f831991fce9a7b20"