shared: move storage-util to libshared

shared: move storage-util to libshared

The storage backend providers (block, fs) and storagectl currently each
extract storage-util.c into their target. Several upcoming consumers
(machine-util's BindVolume parser, vmspawn's hotplug glue, machinectl's
new bind-volume verbs) need the StorageProvider type/string-table
helpers and a future shared Acquire client helper.

Move storage-util.{c,h} to src/shared so libshared exports the symbols
once and every consumer (storage providers, storagectl, libshared
itself) picks them up by linking libshared. Drop the now-redundant
'extract'/'objects' wiring in src/storage/meson.build.

No code changes; this is purely a relocation.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>