Refactor MigrationsModelDiffer to allow third-party changes

[stanislav-a-frolov]

Hello, everyone.

We're using EfCoreTriggers together with our own modified MigrationsModelDiffer. Right now we have to subclass EfCoreTriggers MigrationsModelDiffer since migration code is fully inside GetDifferences. If migrations were implemented via some kind of mixin class that would use ITriggerVisitor to generate and merge migrations operations so it could be chained with another migration modifications.

Made a PR: https://github.com/win7user10/Laraue.EfCoreTriggers/pull/76