Introduce interface for parts of AbstractMigration that a user can depend on

Right now AbstractMigration has some protected methods and methods that we might not to want users to be able to depend on. Lets create an interface to define which methods the user can actually depend on BC.

/cc @Majkl578

Start by locating AbstractMigration and reviewing its protected and other methods to determine which are intended as user-facing BC guarantees. Done means an interface clearly defines only the methods users may depend on, with the relevant compatibility behavior covered by the project’s tests.

Written by the indexing model from the issue text.