[keyserver] convert migrations from map of tuples to list of objects

Summary: This converts our tuples used to specify a migration to an object. This will be used later to specify a migrationType which we can use to determine if downtime is necessary for secondary nodes.

Test Plan: Created a new dummy migration and ran the migration locally

Reviewers: varun, ashoat

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13208