Add PaLM2 API support to JS SDK

[qdwg]

Summary

• adds a Palm2AI client for Google PaLM/Text Bison generateText calls
• exports Palm2AI from the AI package entrypoint
• adds a jsonnet-based PaLM2 example and unit coverage for default/custom request options

Tests

• npm run build
• npx vitest run palm2.test.ts

Fixes #279

[github-actions]

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[qdwg]

I have read the Arakoo CLA Document and I hereby sign the CLA

[qdwg]

recheck

[qdwg]

/claim #279

This PR is ready for review. I signed the Arakoo CLA, and the CLA bot has commented that all contributors have signed. The CLAAssistant check still appears to show the earlier failed run, so could you please re-run the CLA/checks when you get a chance?

Verification:

• npm run build
• npx vitest run palm2.test.ts