Manage the litellm plugin via config/plugins (import existing registration)

Bring the litellm plugin under terraform management like the gpg one. It was
registered manually before terraform owned the catalog, so its state must be
imported before applying, otherwise apply fails creating an entry that already
exists.

- Add config/plugins/vault-plugin-secrets-litellm.yaml (sha256 = released v0.1.1
  openbao binary; verify against the live catalog on import).

Manual pre-step before apply:
  cd environments/au/syd1
  terragrunt import \
    'module.plugin["vault-plugin-secrets-litellm"].vault_plugin.this' \
    secret/vault-plugin-secrets-litellm

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

@@ -0,0 +1,13 @@
+# config/plugins/vault-plugin-secrets-litellm.yaml
+# Imports (registers) the litellm secrets plugin in the catalog. This plugin was
+# registered manually before terraform managed the catalog, so its state must be
+# imported before the first apply (see the PR description) — otherwise apply
+# tries to create an entry that already exists.
+#
+# sha256 is the released v0.1.1 openbao binary
+# (openbao-plugin-secrets-litellm RPM -> /opt/openbao-plugins/vault-plugin-secrets-litellm),
+# which Puppet installs floating. Verify against the live catalog during import
+# (`bao read sys/plugins/catalog/secret/vault-plugin-secrets-litellm`).
+type: secret
+command: vault-plugin-secrets-litellm
+sha256: "2263ebcb3498877a87ddcf31a9cbc6efca6b81702a5faecf7fe7e40200ca7a1f"