Add an objstorage checker to detect missing and corrupted contents

Add class ObjectStorageChecker to detect missing and corrupted contents in an object storage.

It iterates on content objects referenced in a storage instance, check they are available in a given object storage instance then retrieve their bytes from it in order to recompute checksums and detect corruptions.

Some refactorings have also been done in a first commit to extract checkers common code and features in abstract base classes.

Depends on swh-objstorage!177 (merged).

Related to #4694 (closed).