feat(js): add rerankers

[cabljac]

This PR adds rerankers as an action. It also adds an example reranker to the vertexai plugin.

resolves https://github.com/firebase/genkit/issues/771

Checklist (if applicable):

• [x] Tested - only manually tested so far (manually, unit tested, etc.)
• [ ] Changelog updated
• [ ] Docs updated

I will add tests and docs once this design is confirmed and approved.