Clean separation of schema migration

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.

Start by tracing the schema migration functionality across the States API, platform code, and hedera-app-specific code. Done means generic migration code remains cleanly separated in the States API while the app-specific functionality remains available to hedera-app.

Written by the indexing model from the issue text.