LDK: Migrate from VSS to Sqlite storage

[rolznz]

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).