LDK: Migrate from VSS to Sqlite storage

VSS requires a paid Alby Account. Some users may want to migrate to self-hosted and not use an Alby Account.

There is a workaround right now: close all your channels or use hub-recovery (does the same) and then start fresh with the same seed (or a brand new seed and use something like Boltz to migrate the funds into the new wallet).

It should be possible to implement a similar migration from VSS -> Sqlite. It's a bit more complicated (to clear all keys for the user's token in the VSS database after migrating, in the case they migrate back to VSS we will not have corrupt state).

No files or tests are named. Start by locating the VSS and SQLite storage implementations and tracing how channel state and user-token keys are persisted; define the migration flow and its rollback behavior before changing anything. Done means existing VSS state can be migrated to SQLite without closing channels, and the VSS keys are cleared safely so migrating back does not corrupt state.

Written by the indexing model from the issue text.