feat(js/plugins/google-genai): serviceTier support for googleAI

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request introduces the serviceTier parameter to the Google GenAI plugin, allowing users to specify 'standard', 'flex', or 'priority' tiers. The changes update the GenerateContentRequest type, the GeminiConfigSchema, and the request logic, while adding corresponding tests and a sample flow. A review comment suggests relaxing the serviceTier schema validation to a union of the enum and a string to ensure forward compatibility with future API updates.