callbacks: expose DEFAULT_CALLBACK

[skshetry]

Since this is used in function signatures in put/get/get_file/put_file, which could be used in other implementations, there is no point in hiding DEFAULT_CALLBACK.

[martindurant]

This is fine, but I would have argued that _ does not mean "hidden" but "not meant to be called from outside the package".

[skshetry]

This is fine, but I would have argued that _ does not mean "hidden" but "not meant to be called from outside the package".

The _DEFAULT_CALLBACK leaves a bad taste IMO, and I don't see a strong reason to discourage it's use. We have a lot of wrapper functions for cross-filesystem transfer operations that use DEFAULT_CALLBACK to match the fsspec.

I don't have a strong opinion. I'll leave the decision up to you.