routing: swap out final hop blinded route pub keys

If multiple blinded paths are provided, they will each have a different
pub key for the destination node. This makes using our existing
pathfinding logic tricky since it depends on having a single destination
node (characterised by a single pub key). We want to re-use this logic.
So what we do is swap out the pub keys of the destinaion hop with a
pseudo target pub key. This will then be used during pathfinding. Later
on once a path is found, we will swap the real destination keys back in
so that onion creation can be done.

routing/router_test.go

@@ -2231,7 +2231,10 @@ func TestNewRouteRequest(t *testing.T) {
 		t.Run(testCase.name, func(t *testing.T) {
 			t.Parallel()
 
-			var blindedPathInfo *BlindedPaymentPathSet
+			var (
+				blindedPathInfo *BlindedPaymentPathSet
+				expectedTarget  = testCase.expectedTarget
+			)
 			if testCase.blindedPayment != nil {
 				blindedPathInfo, err = NewBlindedPaymentPathSet(
 					[]*BlindedPayment{
@@ -2239,6 +2242,10 @@ func TestNewRouteRequest(t *testing.T) {
 					},
 				)
 				require.NoError(t, err)
+
+				expectedTarget = route.NewVertex(
+					blindedPathInfo.TargetPubKey(),
+				)
 			}
 
 			req, err := NewRouteRequest(
@@ -2253,7 +2260,7 @@ func TestNewRouteRequest(t *testing.T) {
 				return
 			}
 
-			require.Equal(t, req.Target, testCase.expectedTarget)
+			require.Equal(t, req.Target, expectedTarget)
 			require.Equal(
 				t, req.FinalExpiry, testCase.expectedCltv,
 			)