Add Milestone Logic

What does this MR do and why?

Please see issue #414014 (closed) for wider context and !128144 (closed) for a preview of how this MR's code will fit into the overall feature.

In order to introduce the substantial feature of changing migration run order to follow milestones, this is the preliminary scaffolding. This code does not currently run anywhere in production. In future MRs we will require new migrations to specify a milestone, however we are not creating migrations with milestones yet.

How to set up and validate locally

All behavior is currently validated by spec testing.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.