Add begin_interactive_funding_tx_construction()

Author: optout21

lightning/src/ln/channel.rs

@@ -4994,7 +4994,6 @@ pub(super) struct DualFundingChannelContext {
 	/// The amount in satoshis we will be contributing to the channel.
 	pub our_funding_satoshis: u64,
 	/// The amount in satoshis our counterparty will be contributing to the channel.
-	#[allow(dead_code)] // TODO(dual_funding): Remove once contribution to V2 channels is enabled.
 	pub their_funding_satoshis: Option<u64>,
 	/// The funding transaction locktime suggested by the initiator. If set by us, it is always set
 	/// to the current block height to align incentives against fee-sniping.