Two-stage migrations (compatibility/staging mode)

[AndriySvyryd]

Add a way of generating separate scripts for just the non-breaking changes and for the breaking changes. This would allow the previous version of the application to continue to work against the database while the new version is being deployed/tested.
The pattern is sometimes called “Expand/Contract Migrations” because it groups migrations into two categories: Those that are non-destructive expansions of the DB schema and those that are destructive contractions of the schema.

[ajcvickers]

@AndriySvyryd Can you add a bit more explanation, like what we did in triage.

[AndriySvyryd]

It's currently possible to do it manually by carefully planning out the changes. As the first improvement we could make this easier by adding a flag to validate that the new migration has no breaking changes.

[bricelam]

Possibly covered by dotnet/EntityFramework.Docs#691

[ajcvickers]

We discussed this again and decided that, beyond guidance (possibly a case study) there is nothing we're going to do here.

[ajcvickers]

Also, per triage, pinging @divega