[services][backup] LogItem size utilities

Summary:
We need a way to determine if given log item doesn't exceed DynamoDB item limit. See C++ counterpart.

This probably isn't the best solution as stated in https://linear.app/comm/issue/ENG-1336/avoid-hardcoding-database-items-fields-names but for now I'm trying to keep parity with the C++ way.

Depends on D6199

Test Plan: These will be tested with the AddAttachments and SendLog endpoints. I'm wondering if adding unit tests is worth doing here.

Reviewers: varun, tomek, jon, max

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6203