Add a plugin-import module + config/plugins for catalog registration

Registering a plugin in the catalog is a distinct concern from mounting an
engine, so give it its own module and config directory instead of folding
sha256/command into each engine's backend module.

- Add a generic 'plugin' module (vault_plugin: type/name/command/sha256/
  plugin_version) that imports a binary into the catalog.
- Add a config/plugins/ discovery group (filename = catalog name) and wire it
  through vault_cluster (plugins variable + module) and the syd1 environment.
- Add config/plugins/vault-plugin-secrets-gpg.yaml pinning the released v0.1.0
  binary sha256. Other engines can now register their plugin by dropping a file
  here; their *_secret_backend module just mounts the registered type.

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"