build: register librarian repo for automated releases (#1883)

Author: chingor13

internal/automation/prod/repositories.yaml

@@ -13,3 +13,9 @@ repositories:
       - generate
       - stage-release
       - publish-release
+  - name: "librarian"
+    full-name: https://github.com/googleapis/librarian
+    github-token-secret-name: "librarian-github-token"
+    supported-commands:
+      - stage-release
+      - publish-release