storage: add systemd-storage-block@.service provider

storage: add systemd-storage-block@.service provider

First implementation of io.systemd.StorageProvider, exposing all block
devices known to udev (disks, partitions, dm nodes, …) as volumes of
type "blk". Names are picked from stable /dev/mapper and /dev/disk/by-*
symlinks; content-derived identifiers (by-uuid, by-label, …) are
intentionally avoided for security. Volume creation is not supported by
this backend.

Socket-activated via /run/systemd/io.systemd.StorageProvider/block.
Also adds shared storage-util.[ch] (VolumeType / CreateMode helpers)
that subsequent providers reuse.