Clean separation of schema migration

[netopyr]

When we moved the States API to platform, we also moved the schema migration functionality, as it was needed by platform code. However, we did not modify the code, which was specific to hedera-app.

We need a clean separation of the generic code that should continue to live in the States API and app-specific code. The schema migration functionality should be redesigned to allow this clean separation while making the app-specific code available.