feat(js/plugins/google-genai): PayGo support for priority and flex

[ifielker]

Sample code:

const response = await ai.generate({
    model: vertexAI.model('gemini-3.1-flash-lite-preview'),
    prompt:
      'You are a helpful AI assistant named Walt, say hello.',
    config: {
      payGo: 'priority', // or priority-only, flex, flex-only.
    },
  });

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 adds support for PayGo consumption options to the Vertex AI Gemini plugin, introducing a payGo configuration field that maps to specific request headers. It also enables customHeaders in the client and adds a trafficType field to UsageMetadata. Feedback indicates that the trafficType field is not yet mapped in the model response usage object, which should be addressed to ensure the data is correctly captured.