[services] Backup - Update add attachments logic

Summary:
Depends on D4322

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

Every time we add attachments to the log item, we check if now they are still small enough to be kept in the database, and if they're not, we move their data to the S3.

Test Plan: Services build

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phab.comm.dev/D4323