feat(js/plugins/ollama): implemented dynamic model resolver and list actions for ollama plugin

[pavelgj]

a few other changes:

made plugin options optional:

const ai = genkit({plugins:[ollama()]})

also added ollama.model and ollama.embedder helper functions:

ai.generate({
  model: ollama.model('gemma3:latest')
})

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)