feat(js/plugins/vertexai): instruduced gemini model ref helper and ability to register versions

[pavelgj]

gemini model ref helper, consistent with googleAI plugin.

import { vertexAI, gemini } from '@genkit-ai/vertexai';

const gemini15Flash042 = gemini('gemini-1.5-flash-042');
const ai = genkit({
  plugins: [
    vertexAI({
      models: [gemini15Flash042]
    })
  ],
  model: gemini15Flash042
});

await generate('hi')

Checklist (if applicable):

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