Expose on-chain transactions in events

[tnull]

In #432 we exposed on-chain transactions in the payment store.

We should also do this in events, although for the latter I'd prefer to wait until we can filter out channel-related transactions properly to avoid redudant events, e.g., on channel closure.

We also might want to wait until BDK finally includes on-chain events, which might allow us to make the logic a bit more lightweight.

Blocked on bitcoindevkit/bdk#1452 and lightningdevkit/rust-lightning#3548