Return FileOutput from `run()` function

This is currently behind the `useFileOutput` flag provided to the
Replicate constructor. This allows us to test the feature before rolling
it out more widely.

When enabled any URLs or data-uris will be converted into a FileOutput
type. This is essentially a `ReadableStream` that has two additional
methods `url()` to return the underlying URL and `blob()` which will
return a `Blob()` object with the file data loaded into memory.

The intention here is to make it easier to work with file outputs and
allows us to optimize the delivery of file assets to the client in
future iterations.