fs: merge upload_fobj and upload

[skshetry]

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[efiop]

@skshetry Btw, could you clarify how you see it in relation to fsspec?

[skshetry]

I don't think I have any motivations to unify them from fsspec perspective. My first preference there would be to have an explicit API like upload_fileobj/upload_fobj and my second choice would be to extend pipe_file to support file objects.
upload and upload_fobj are quite different things in the filesystem layer.

For us, what we need is a fs.utils.write like method (discussed in #6558 (comment)) that can also provide callback support, which is more of a service layer for fs.

I am assuming upload_fobj as a fsspec's API at least for now, so I have tried to not add callback support there, as the caller can do so themselves quite easily.

I'll consider upload API to be that fs.utils.write, that extends fsspec API for us, rather than being fsspec-related.