Add custom TLV support to PendingOutboundPayment for Bolt12 flows

Bolt11 payments now allow attaching custom TLVs to the onion packet,
giving payers a way to pass metadata ranging from descriptions to
custom authentication. Bolt12 lacked an equivalent path, leaving its
outbound flow less extensible.

This commit adds a `custom_tlvs` field to the Bolt12-related
`PendingOutboundPayment` variants, bringing the internal Bolt12
payment state in line with the Bolt11 changes and preparing the
ground for user-facing API support.

A follow-up commit will expose a way for users to set these TLVs.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -12888,7 +12888,7 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 		let expiration = StaleExpiration::AbsoluteTimeout(absolute_expiry);
 		$self.pending_outbound_payments
 			.add_new_awaiting_invoice(
-				payment_id, expiration, retry_strategy, route_params_config, None,
+				payment_id, vec![], expiration, retry_strategy, route_params_config, None,
 			)
 			.map_err(|_| Bolt12SemanticError::DuplicatePaymentId)?;
 
@@ -12932,7 +12932,7 @@ macro_rules! create_refund_builder { ($self: ident, $builder: ty) => {
 		let expiration = StaleExpiration::AbsoluteTimeout(absolute_expiry);
 		$self.pending_outbound_payments
 			.add_new_awaiting_invoice(
-				payment_id, expiration, retry_strategy, route_params_config, None,
+				payment_id, vec![], expiration, retry_strategy, route_params_config, None,
 			)
 			.map_err(|_| Bolt12SemanticError::DuplicatePaymentId)?;
 
@@ -13054,6 +13054,7 @@ where
 			self.pending_outbound_payments
 				.add_new_awaiting_invoice(
 					payment_id,
+					vec![],
 					StaleExpiration::TimerTicks(1),
 					optional_params.retry_strategy,
 					optional_params.route_params_config,
@@ -13083,6 +13084,7 @@ where
 			self.pending_outbound_payments
 				.add_new_awaiting_invoice(
 					payment_id,
+					vec![],
 					StaleExpiration::TimerTicks(1),
 					optional_params.retry_strategy,
 					optional_params.route_params_config,
@@ -13125,6 +13127,7 @@ where
 			self.pending_outbound_payments
 				.add_new_awaiting_invoice(
 					payment_id,
+					vec![],
 					StaleExpiration::TimerTicks(1),
 					optional_params.retry_strategy,
 					optional_params.route_params_config,
@@ -13288,6 +13291,7 @@ where
 		let expiration = StaleExpiration::TimerTicks(1);
 		self.pending_outbound_payments.add_new_awaiting_offer(
 			payment_id,
+			vec![],
 			expiration,
 			optional_params.retry_strategy,
 			optional_params.route_params_config,