lnrpc: allow payment addr override for AMP invoices

This permits an AMP invoice to be "pseudo-reusable", where the invoice
paramters can be used multiple times so long as a new payment address is
supplied. This prevents additional round trips between payer and payee
to obtain a new invoice, even though the payments/invoices won't be
logically associated via the RPC interface like they would when the full
reusable invoices are deployed.

lnrpc/routerrpc/router_backend.go

@@ -710,6 +710,7 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 			payIntent.MaxParts = 1
 		}
 
+		payAddr := payReq.PaymentAddr
 		if payReq.Features.HasFeature(lnwire.AMPOptional) {
 			// Generate random SetID and root share.
 			var setID [32]byte
@@ -730,6 +731,17 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 			if err != nil {
 				return nil, err
 			}
+
+			// For AMP invoices, we'll allow users to override the
+			// included payment addr to allow the invoice to be
+			// pseudo-reusable, e.g. the invoice parameters are
+			// reused (amt, cltv, hop hints, etc) even though the
+			// payments will share different payment hashes.
+			if len(rpcPayReq.PaymentAddr) > 0 {
+				var addr [32]byte
+				copy(addr[:], rpcPayReq.PaymentAddr)
+				payAddr = &addr
+			}
 		} else {
 			err = payIntent.SetPaymentHash(*payReq.PaymentHash)
 			if err != nil {
@@ -745,7 +757,7 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 			payIntent.RouteHints, payReq.RouteHints...,
 		)
 		payIntent.DestFeatures = payReq.Features
-		payIntent.PaymentAddr = payReq.PaymentAddr
+		payIntent.PaymentAddr = payAddr
 		payIntent.PaymentRequest = []byte(rpcPayReq.PaymentRequest)
 	} else {
 		// Otherwise, If the payment request field was not specified