chore: add librarian.yaml to google-cloud-java

[sofisl]

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

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[zhumin8]

Note: it is safe to merge this in even when we want to use hermetic build.
Running librarian generate is manual and does not affect the existing hermetic build workflows. If this librarian.yaml gets out of sync with generation_config.yaml during migration, we will do steps in the description again and update it.