[lib] Move transforming rows into updates to specs

Summary:
Move the logic into specs.

https://linear.app/comm/issue/ENG-4238/use-specs-in-update-fetchers

Test Plan: Generate some updates and check if they are fetched from the db.

Reviewers: kamil, bartek, inka

Reviewed By: bartek

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9442