objstorage: allow use of file-like objects for streaming methods

Currently the objstorage {get,set}_stream methods take and return an iterator. It would be way better to have them also take and return a file-like object, that could be written like this:

https://stackoverflow.com/questions/12593576/adapt-an-iterator-to-behave-like-a-file-like-object-in-python/12604375#12604375

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Migrated from T805 (view on Phabricator)

mentioned in issue swh-vault#928 (closed)

assigned to @seirl

added Object storage priority:Triage labels

added priority:Normal label and removed priority:Triage label

marked this issue as related to swh-vault#885 (closed)

unassigned @seirl