Create new `PaymentKind` variants for channel funding/closing on-chain transactions

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

Once we're able to discern channel-related transactions, we should create dedicated `PaymentKind`s for them and use `Onchain` only for payments/as a catchall

Blocked on  https://github.com/lightningdevkit/rust-lightning/issues/3548