feat(js/plugins/{googleai,vertexai}): implemented dynamic model listing for googleai, and new model lookup API for googleai & vertexai

[pavelgj]

model listing for vertexai is coming next (more work than expected).

new model lookup API:

import { googleAI } from '@genkit-ai/googleai';

await ai.generate({
  prompt: '...',
  model: googleAI.model('gemini-2.0-flash-001')
})
// can set config right on the model ref
googleAI.model('gemini-2.0-flash-001', { temperature: 0.7 })

similarly for vertexAI

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

await ai.generate({
  prompt: '...',
  model: vertexAI.model('gemini-2.0-flash-001')
})
// can set config right on the model ref
vertexAI.model('gemini-2.0-flash-001', { temperature: 0.7 })

similarly for embedders:

googleAI.embedder('text-embedder-004')
vertexAI.embedder('text-embedder-004')

models are dynamically listed in the dev ui, pulled from the plugin's listActions

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)