Support encoding invreqs in outbound onion payloads

Per BOLTs PR 1149, when paying a static invoice we need to include our original
invoice request in the HTLC onion since the recipient wouldn't have received it
previously.

We use an experimental TLV type for this new onion payload field, since the
spec is still not merged in the BOLTs.

lightning/src/ln/max_payment_path_len_tests.rs

@@ -184,6 +184,7 @@ fn one_hop_blinded_path_with_custom_tlv() {
 		encrypted_tlvs: &blinded_path.blinded_hops()[0].encrypted_payload,
 		intro_node_blinding_point: Some(blinded_path.blinding_point()),
 		keysend_preimage: None,
+		invoice_request: None,
 		custom_tlvs: &Vec::new()
 	}.serialized_length();
 	let max_custom_tlv_len = 1300

lightning/src/ln/msgs.rs

@@ -1747,6 +1747,7 @@ pub struct FinalOnionHopData {
 mod fuzzy_internal_msgs {
 	use bitcoin::secp256k1::PublicKey;
 	use crate::blinded_path::payment::{BlindedPaymentPath, PaymentConstraints, PaymentContext, PaymentRelay};
+	use crate::offers::invoice_request::InvoiceRequest;
 	use crate::types::payment::{PaymentPreimage, PaymentSecret};
 	use crate::types::features::{BlindedHopFeatures, Bolt12InvoiceFeatures};
 	use super::{FinalOnionHopData, TrampolineOnionPacket};
@@ -1827,6 +1828,7 @@ mod fuzzy_internal_msgs {
 			intro_node_blinding_point: Option<PublicKey>, // Set if the introduction node of the blinded path is the final node
 			keysend_preimage: Option<PaymentPreimage>,
 			custom_tlvs: &'a Vec<(u64, Vec<u8>)>,
+			invoice_request: Option<&'a InvoiceRequest>,
 		}
 	}
 
@@ -2760,13 +2762,17 @@ impl<'a> Writeable for OutboundOnionPayload<'a> {
 			},
 			Self::BlindedReceive {
 				sender_intended_htlc_amt_msat, total_msat, cltv_expiry_height, encrypted_tlvs,
-				intro_node_blinding_point, keysend_preimage, ref custom_tlvs,
+				intro_node_blinding_point, keysend_preimage, ref invoice_request, ref custom_tlvs,
 			} => {
 				// We need to update [`ln::outbound_payment::RecipientOnionFields::with_custom_tlvs`]
 				// to reject any reserved types in the experimental range if new ones are ever
 				// standardized.
+				let invoice_request_tlv = invoice_request.map(|invreq| (77_777, invreq.encode())); // TODO: update TLV type once the async payments spec is merged
 				let keysend_tlv = keysend_preimage.map(|preimage| (5482373484, preimage.encode()));
-				let mut custom_tlvs: Vec<&(u64, Vec<u8>)> = custom_tlvs.iter().chain(keysend_tlv.iter()).collect();
+				let mut custom_tlvs: Vec<&(u64, Vec<u8>)> = custom_tlvs.iter()
+					.chain(invoice_request_tlv.iter())
+					.chain(keysend_tlv.iter())
+					.collect();
 				custom_tlvs.sort_unstable_by_key(|(typ, _)| *typ);
 				_encode_varint_length_prefixed_tlv!(w, {
 					(2, HighZeroBytesDroppedBigSize(*sender_intended_htlc_amt_msat), required),

lightning/src/ln/onion_utils.rs

@@ -262,6 +262,7 @@ where
 								encrypted_tlvs: &blinded_hop.encrypted_payload,
 								intro_node_blinding_point: blinding_point.take(),
 								keysend_preimage: *keysend_preimage,
+								invoice_request: None,
 								custom_tlvs: &recipient_onion.custom_tlvs,
 							},
 						);

lightning/src/ln/outbound_payment.rs

@@ -641,6 +641,7 @@ impl RecipientOnionFields {
 		for (typ, _) in custom_tlvs.iter() {
 			if *typ < 1 << 16 { return Err(()); }
 			if *typ == 5482373484 { return Err(()); } // keysend
+			if *typ == 77_777 { return Err(()); } // invoice requests for async payments
 			match prev_type {
 				Some(prev) if prev >= *typ => return Err(()),
 				_ => {},