routing: introduce routing hints to lightning payments

In this commit, we add a new field of routing hints to payments over the
Lightning Network. These routing hints can later be used within the path
finding algorithm in order to craft a path that will reach the
destination succesfully.

routing/pathfind.go

@@ -28,6 +28,28 @@ const (
 	infinity = math.MaxInt64
 )
 
+// HopHint is a routing hint that contains the minimum information of a channel
+// required for an intermediate hop in a route to forward the payment to the
+// next. This should be ideally used for private channels, since they are not
+// publicly advertised to the network for routing.
+type HopHint struct {
+	// NodeID is the public key of the node at the start of the channel.
+	NodeID *btcec.PublicKey
+
+	// ChannelID is the unique identifier of the channel.
+	ChannelID uint64
+
+	// FeeBaseMSat is the base fee of the channel in millisatoshis.
+	FeeBaseMSat uint32
+
+	// FeeProportionalMillionths is the fee rate, in millionths of a
+	// satoshi, for every satoshi sent through the channel.
+	FeeProportionalMillionths uint32
+
+	// CLTVExpiryDelta is the time-lock delta of the channel.
+	CLTVExpiryDelta uint16
+}
+
 // ChannelHop is an intermediate hop within the network with a greater
 // multi-hop payment route. This struct contains the relevant routing policy of
 // the particular edge, as well as the total capacity, and origin chain of the

routing/router.go

@@ -1482,6 +1482,16 @@ type LightningPayment struct {
 	// indefinitely.
 	PayAttemptTimeout time.Duration
 
+	// RouteHints represents the different routing hints that can be used to
+	// assist a payment in reaching its destination successfully. These
+	// hints will act as intermediate hops along the route.
+	//
+	// NOTE: This is optional unless required by the payment. When providing
+	// multiple routes, ensure the hop hints within each route are chained
+	// together and sorted in forward order in order to reach the
+	// destination successfully.
+	RouteHints [][]HopHint
+
 	// TODO(roasbeef): add e2e message?
 }