chore(librarian): onboard to librarian (#549)

Towards googleapis/librarian#2451

Author: parthea

.github/release-please.yml

@@ -0,0 +1,10 @@
+image: us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-prod/python-librarian-generator@sha256:8e2c32496077054105bd06c54a59d6a6694287bc053588e24debe6da6920ad91
+libraries:
+  - id: proto-plus
+    version: 1.26.1
+    apis: []
+    source_roots:
+      - .
+    preserve_regex: []
+    remove_regex: []
+    tag_format: v{version}

.github/release-trigger.yml

@@ -1,5 +1,9 @@
 # Changelog
 
+[PyPI History][1]
+
+[1]: https://pypi.org/project/proto-plus/#history
+
 ## [1.26.1](https://github.com/googleapis/proto-plus-python/compare/v1.26.0...v1.26.1) (2025-03-05)