Add Batch.is_s3() helper

Guessing the upload provider from outside the client is something like:

if object.batch.get("provider", "") == "s3":
    # ...

Not very intuitive nor pratical.

The new method, Batch.is_s3(), will simply do:

return self.provider == "s3"

It will be used in Nuxeo Drive directly and can be useful to anyone implementing uploads using that client.