feat: allow specifying middleware on the generate function

[pavelgj]

The idea is to be able to do things like

await generate({
  prompt: prompt,
  model: model,
  use: [
    retry({ maxRetries: 2 }),
    smoothStreaming({ cadence: 25, randomize: true })
  ],
})

[pavelgj]

Nope... this is bad. Should not be allowed!!!