feat: response options as function

[AndersDJohnson]

We may be able to support providing a function to generate entire response options - similar to how we support functions for some response option keys like json, text, latency, status, etc. This would enable a few more complex behaviors, e.g., allow us to conditionally return different content types:

mockyeah.get('/hi', (req, res) => {
  if (req.query.format === 'text') {
    return {
      text: 'bar'
    }
  } else {
    return {
      json: { bar: true }
    }
  }
})

Relates to #530.

Not sure how we would still support the name property on response options without always calling the function - especially on misses where we wouldn't normally call this. Maybe rely on the function .name property?