Add pending funding scopes to FundedChannel

Once a channel is funded, it may be spliced to add or remove funds. The
new funding transaction is pending until confirmed on chain and thus
needs to be tracked. Additionally, it may be replaced by another
transaction using RBF with a higher fee. Hence, there may be more than
one pending FundingScope to track for a splice.

This commit adds support for tracking pending funding scopes. The
following commits will account for any pending scopes where applicable
(e.g., when handling commitment_signed).

Author: jkczyz

lightning/src/ln/channel.rs

@@ -1513,6 +1513,7 @@ impl<SP: Deref> Channel<SP> where
 				};
 				let mut funded_channel = FundedChannel {
 					funding: chan.funding,
+					pending_funding: vec![],
 					context: chan.context,
 					interactive_tx_signing_session: chan.interactive_tx_signing_session,
 					holder_commitment_point,
@@ -4820,6 +4821,7 @@ pub(super) struct DualFundingChannelContext {
 // Counterparty designates channel data owned by the another channel participant entity.
 pub(super) struct FundedChannel<SP: Deref> where SP::Target: SignerProvider {
 	pub funding: FundingScope,
+	pending_funding: Vec<FundingScope>,
 	pub context: ChannelContext<SP>,
 	pub interactive_tx_signing_session: Option<InteractiveTxSigningSession>,
 	holder_commitment_point: HolderCommitmentPoint,
@@ -9278,6 +9280,7 @@ impl<SP: Deref> OutboundV1Channel<SP> where SP::Target: SignerProvider {
 
 		let mut channel = FundedChannel {
 			funding: self.funding,
+			pending_funding: vec![],
 			context: self.context,
 			interactive_tx_signing_session: None,
 			is_v2_established: false,
@@ -9545,6 +9548,7 @@ impl<SP: Deref> InboundV1Channel<SP> where SP::Target: SignerProvider {
 		// `ChannelMonitor`.
 		let mut channel = FundedChannel {
 			funding: self.funding,
+			pending_funding: vec![],
 			context: self.context,
 			interactive_tx_signing_session: None,
 			is_v2_established: false,
@@ -10785,6 +10789,7 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, &'c Channel
 				channel_transaction_parameters: channel_parameters,
 				funding_transaction,
 			},
+			pending_funding: vec![],
 			context: ChannelContext {
 				user_id,