[Services] Update backup CMake

Summary:
Make backup more nix friendly
Allow backup to be built outside of docker

Also moved Item.h to shared lib

Test Plan:

nix develop
cd services/backup && mkdir build && cd build && cmake .. && make

docker workflow:

cd services/base-image
docker build -t commapp/services-base . -f Dockerfile

cd ../..
docker build . -f services/backup/Dockerfile

Reviewers: max, karol, atul

Reviewed By: atul

Subscribers: ashoat, tomek, adrian, atul, abosh, karol, max

Differential Revision: https://phab.comm.dev/D4494