[services] Backup - Send Log - Check the real item size

Summary:
Depends on D4411

https://linear.app/comm/issue/ENG-1288/properly-handle-the-dynamodb-item-size-limit

Test Plan: Services build, for now this went out of sync with the integration tests

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phab.comm.dev/D4412