[terraform] Add Backup to ECS Service Connect

Summary:
Needed to make Backup service discoverable in ECS Service Connect, to let Identity reach it when deleting user data.

Depends on D12972

Test Plan: terraform plan + tested on staging

Reviewers: will, varun, kamil

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12974