LSPS2: Emit event when fee has been collected for a channel

[johncantrell97]

Once PaymentForwarded has the skimmed_fee_msat parameter in LDK 123 we should add an event to notify the LSP server that the fee has been paid for a channel.

Otherwise, the LSP needs to duplicate this logic to decide when to broadcast the funding transaction if using client trusts lsp.

[johncantrell97]

I think we can do this without an event and just return information from payment_forwarded event instead.