Add support for slices in Storage.content_get

Currently Storage.content_get only supports returning the full blob. We should add new parameters (eg. start and end) to specify the range of bytes the caller wants, and pass them to the objstorage.

Note that both swh/storage/storage.py and swh/storage/in_memory.py should be modified.

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Migrated from T1446 (view on Phabricator)