Remove `keychain::ChangeSet::keychains_added` and add this to `bdk_wallet::ChangeSet`

[notmandatory]

Sub-task for #1103

[gnapoli23]

@notmandatory Can I pick this one? It should be just a refactor right? If ok I would go for it.

[notmandatory]

@gnapoli23 thanks for offering to pitch in on this, I need to check with @evanlinjin and make sure he's not doing this in the PR he's working on.

[gnapoli23]

All right, ok please let me know @notmandatory . Thanks.

[evanlinjin]

Sorry I'm already working on this.