Extend begin_interactive_...() with splicing-specific parameters

The begin_interactive_funding_tx_construction() method is extended with
`is_initiator` parameter (splice initiator), and
`prev_funding_input` optional parameter, containing the previous funding
transaction, which will be added to the negotiation as an input by the
initiator.

Author: optout21

lightning/src/ln/channel.rs

@@ -2706,7 +2706,8 @@ where
 	#[rustfmt::skip]
 	fn begin_interactive_funding_tx_construction<ES: Deref>(
 		&mut self, signer_provider: &SP, entropy_source: &ES, holder_node_id: PublicKey,
-		change_destination_opt: Option<ScriptBuf>,
+		is_initiator: bool, change_destination_opt: Option<ScriptBuf>,
+		prev_funding_input: Option<(TxIn, TransactionU16LenLimited)>,
 	) -> Result<Option<InteractiveTxMessageSend>, AbortReason>
 	where ES::Target: EntropySource
 	{
@@ -2716,7 +2717,11 @@ where
 		let mut funding_inputs = Vec::new();
 		mem::swap(&mut self.funding_negotiation_context.our_funding_inputs, &mut funding_inputs);
 
-		// TODO(splicing): Add prev funding tx as input, must be provided as a parameter
+		if is_initiator {
+			if let Some(prev_funding_input) = prev_funding_input {
+				funding_inputs.push(prev_funding_input);
+			}
+		}
 
 		// Add output for funding tx
 		// Note: For the error case when the inputs are insufficient, it will be handled after
@@ -2729,7 +2734,7 @@ where
 			script_pubkey: self.funding.get_funding_redeemscript().to_p2wsh(),
 		};
 
-		if self.funding.is_outbound() {
+		if is_initiator {
 			funding_outputs.push(
 				OutputOwned::Shared(SharedOwnedOutput::new(
 					shared_funding_output, self.funding_negotiation_context.our_funding_satoshis,
@@ -2748,7 +2753,7 @@ where
 				.map_err(|_err| AbortReason::InternalError("Error getting destination script"))?
 		};
 		let change_value_opt = calculate_change_output_value(
-			self.funding.is_outbound(), self.funding_negotiation_context.our_funding_satoshis,
+			is_initiator, self.funding_negotiation_context.our_funding_satoshis,
 			&funding_inputs, &funding_outputs,
 			self.funding_negotiation_context.funding_feerate_sat_per_1000_weight,
 			change_script.minimal_non_dust().to_sat(),
@@ -2774,7 +2779,7 @@ where
 			counterparty_node_id: self.context.counterparty_node_id,
 			channel_id: self.context.channel_id(),
 			feerate_sat_per_kw: self.funding_negotiation_context.funding_feerate_sat_per_1000_weight,
-			is_initiator: self.funding.is_outbound(),
+			is_initiator,
 			funding_tx_locktime: self.funding_negotiation_context.funding_tx_locktime,
 			inputs_to_contribute: funding_inputs,
 			outputs_to_contribute: funding_outputs,