Add ObjectDeletionInterface aimed at Storage classes
swh-alter
needs an interface in order to remove objects from the storage
(so we can handle takedown notices).
The chosen interface is optimal from swh-alter
point of view: it
identifies a whole range of objects with different types that can be
removed alongside a given set of origins.
Giving all objects to be removed at once might also help with consistency constraints inside the various storage facilities.
Edited by Jérémy Bobbio (Lunar)