[services] Lib - Move Item.h

Summary:
Depends on D3908

Moving Item.h from both blob/backup to the mutual "lib" space.

Test Plan:
No real changes are applied, just moving stuff around, so services should still build and that's the test plan

cd services

yarn run-blob-service
yarn run-backup-service

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: geekbrother, ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3909