Describe the enhancement

The bdk_wallet::ChangeSet is what the persistence interfaces with. As suggested by @tnull, this structure should be backwards compatible (in case we make changes to this structure).

An easy solution is to make bdk_wallet::ChangeSet a enum:

#[non_exhaustive]
pub enum ChangeSet {
    V0(ChangeSet0),
    V1(ChangeSet1), // in the future this may exist
}

// We always convert to the latest version
impl From<ChangeSet0> for ChangeSet1 { /* TODO */ }