routing: add BlindedPayment() method to AdditionalEdges

Expand the AdditionalEdges interface with a BlindedPayment method. In
upcoming commits, we will want to know if an AdditionalEdge was derived
from a blinded payment or not and we will also need some information
from the blinded payment it was derived from. So we expand the interface
here to avoid needing to do type casts later on. The new method may
return nil if the edge was not derived from a blinded payment.

routing/additional_edge.go

@@ -29,6 +29,11 @@ type AdditionalEdge interface {
 
 	// EdgePolicy returns the policy of the additional edge.
 	EdgePolicy() *models.CachedEdgePolicy
+
+	// BlindedPayment returns the BlindedPayment that this additional edge
+	// info was derived from. It will return nil if this edge was not
+	// derived from a blinded route.
+	BlindedPayment() *BlindedPayment
 }
 
 // PayloadSizeFunc defines the interface for the payload size function.
@@ -60,6 +65,12 @@ func (p *PrivateEdge) IntermediatePayloadSize(amount lnwire.MilliSatoshi,
 	return hop.PayloadSize(channelID)
 }
 
+// BlindedPayment is a no-op for a PrivateEdge since it is not associated with
+// a blinded payment. This will thus return nil.
+func (p *PrivateEdge) BlindedPayment() *BlindedPayment {
+	return nil
+}
+
 // BlindedEdge implements the AdditionalEdge interface. Blinded hops are viewed
 // as additional edges because they are appended at the end of a normal route.
 type BlindedEdge struct {
@@ -120,6 +131,12 @@ func (b *BlindedEdge) IntermediatePayloadSize(_ lnwire.MilliSatoshi, _ uint32,
 	return hop.PayloadSize(0)
 }
 
+// BlindedPayment returns the blinded payment that this edge is associated
+// with.
+func (b *BlindedEdge) BlindedPayment() *BlindedPayment {
+	return b.blindedPayment
+}
+
 // Compile-time constraints to ensure the PrivateEdge and the BlindedEdge
 // implement the AdditionalEdge interface.
 var _ AdditionalEdge = (*PrivateEdge)(nil)