feat(js/plugins/googleai): added support for imagen and veo models to googleai plugin

[pavelgj]

new operation API

  let { operation } = await ai.generate({
    model: googleAI.model('veo-2.0-generate-001'),
    prompt: 'banana riding a bicycle',
  });
  if (!operation) throw new Error('expected operation');

  // or BETA api:
  let operation = await ai.generateOperation({
    model: googleAI.model('veo-2.0-generate-001'),
    prompt: 'banana riding a bicycle',
  });

  while (!operation.done) {
    operation = await ai.checkOperation(operation);
    await new Promise((resolve) => setTimeout(resolve, 5000));
  }

  return operation;

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)