It might be nice to let the bookkeeper keep track of external accounts
as well as the internal onchain wallet? To this end, we add some new
custom notifications, which the bookkeeper will ingest and add to its
ledger.
Suggested-By: @chrisguida
Changelog-Added: PLUGINS: `bookkeeper` now listens for two custom events: `utxo_deposit` and `utxo_spend`. This allows for 3rd party plugins to send onchain coin events to the `bookkeeper`. See the new plugins/bkpr/README.md for details on how these work!
