cnct: parse onion for resolvers

With the introduction of additional payload fields for mpp, it becomes
a necessity to have their values available in the on-chain resolution
flow. The incoming contest resolver notifies the invoice registry of the
arrival of a payment and needs to supply all parameters for the registry
to validate the htlc.

contractcourt/interfaces.go

@@ -1,7 +1,10 @@
 package contractcourt
 
 import (
+	"io"
+
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/htlcswitch/hop"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -26,3 +29,10 @@ type Registry interface {
 	// HodlUnsubscribeAll unsubscribes from all hodl events.
 	HodlUnsubscribeAll(subscriber chan<- interface{})
 }
+
+// OnionProcessor is an interface used to decode onion blobs.
+type OnionProcessor interface {
+	// ReconstructHopIterator attempts to decode a valid sphinx packet from
+	// the passed io.Reader instance.
+	ReconstructHopIterator(r io.Reader, rHash []byte) (hop.Iterator, error)
+}