Add ObjectDeletionInterface aimed at Storage classes

changed milestone to %Tooling for takedown notices [Roadmap - Collect]

assigned to @lunar

The ideal interface for swh-alter would be the following:

class ObjectDeletionInterface(Protocol):
    def object_delete(self, swhids: List[ExtendedSWHID]):
        """Delete objects from the storage

        All skipped content objects matching the given SWHID will be removed,
        including those who have the same SWHID due to hash collisions.

        Origin objects are removed alongside their associated origin visit and
        origin visit status objects.

        Args:
            swhids: list of SWHID of the objects to remove

        Returns:
            Summary dict with the following keys and associated values:

                content:delete: Number of content objects removed
                content:delete:bytes: Sum of the removed contents’ data length
                skipped_content:delete: Number of skipped content objects
                removed directory:delete: Number of directory objects removed
                revision:delete: Number of revision objects removed
                release:delete: Number of release objects removed
                snapshot:delete: Number of snapshot objects removed
                origin:delete: Number of origin objects removed
                origin_visit:delete: Number of origin visit objects removed
                origin_visit_status:delete: Number of origin visit status objects removed
        """
        ...

It feels quite different from the rest of StorageInterface, but maybe this is ok?

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Why have a function for each object type, but then take ExtendedSWHID as argument instead of Sha1Git? What's the meaning of directory_delete(["swh:1:rel:..."])?