chore: add librarian.yaml to google-cloud-java (googleapis#13275)

Steps to produce this PR:

1.
https://docs.google.com/document/d/1moP7zq3Qy7xqjdKSpTJhHbhpX7dSG9Wo38f9nlEpcLQ/edit?resourcekey=0-p1YPSwMTMGFYRgr4jegcOw&tab=t.gxrue4sew3io
2. then run `migrate .`
3. Run librarian generate --all to confirm there are no diffs

Fixes googleapis/librarian#6135

Author: sofisl