chore(librarian): onboard to librarian (#1515)

Author: ohmayr

.librarian/config.yaml

@@ -0,0 +1,6 @@
+global_files_allowlist:
+  # Allow the container to read and write the root `CHANGELOG.md`
+  # file during the `release` step to update the latest client library
+  # versions which are hardcoded in the file.
+  - path: "CHANGELOG.md"
+    permissions: "read-write"

.librarian/state.yaml

@@ -0,0 +1,10 @@
+image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator:latest
+libraries:
+  - id: google-cloud-pubsub
+    version: 2.31.1
+    apis: []
+    source_roots:
+      - .
+    preserve_regex: []
+    remove_regex: []
+    tag_format: v{version}