Add a task ashCheckPersistenceSchemaCompatibility (?) sbt task that would fail on unhandled changes to API

[PawelLipski]

Currently, there's only ashDumpPersistenceSchema and this only dumps the schema to a file, and it's down to the user to verify that there are no changes (when comparing to e.g. develop branch).

This could be automated even more within this plugin, I can see the following steps can be undertaken:

1. the dump can be saved somewhere in the repository (project/ folder? src/<something>/resources/?) and tracked under git — this way the sbt plugin itself will be able to detect the changes, without reliance on e.g. external CI artifacts
2. then, to mark the difference as resolved, the user just needs to replace the existing persistence schema file with the new version (after probably adding necessary schema migrations!)

TBD whether this sbt task should be aware of schema migrations in any way.