[keyserver] Convert raw update infos in a spec

Summary:
Move the logic to a spec.

Depends on D9655

Test Plan: Generate some updates, reintroduce old code, and compare new with old result using _.isEqual

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9656