[services] Backup - Check if log item isn't too big to be stored in the database

Summary:
Depends on D3728

https://linear.app/comm/issue/ENG-725/check-for-the-data-size-and-persistinblob-in-logitems-validate

Test Plan:
It's hard to come up with a certain test plan here as this is just a sanity check. You can always check whether the service builds as usual:

cd services
yarn run-backup-service

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3729