Expose on-chain transactions in events

[tnull]

In bitcoindevkit/bdk#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_wallet#6 and lightningdevkit/rust-lightning#3548

[JeanlChristophe]

Hi,
This feature is crucial for us at Synonym.
We are building a mobile wallet (Bitkit) on top of ldk-node and need reliable on-chain receive events for core user flows:
• Sending proper payment received notifications (today we can only do this for LN payments).
• Updating balance and transaction history in real time.
• Ensuring UX consistency between on-chain and Lightning inside a single app.

Right now, the lack of on-chain events makes it very hard to offer a smooth combined on-chain + Lightning experience. Even a minimal version of this functionality would already unblock critical product features.
We fully understand the need to filter out channel-related transactions later for noise reduction, but from a product perspective, having basic on-chain events exposed as soon as possible would be a big step forward.

Thanks for the great work on both LDK and BDK