Extract the objstorage component from storage module to an independent module

swh.storage.objstorage is growing and is now a more consequent and independent module. It will be extracted from swh.storage into its own module/repository swh.objstorage.

Migrated from T460 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information