multi: add blinded path TLVs to onion payload / hops

This commit adds the encrypted_data, blinding_point and total_amt_msat
tlvs to the known set of even tlvs for the onion payload. These TLVs
are added in two places (the onion payload and hop struct) because
lnd uses the same set of TLV types for both structs (and they
inherently represent the same thing).

Note: in some places, unit tests intentionally mimic the style
of older tests, so as to be more consistently readable.

routing/route/route_test.go

@@ -162,6 +162,8 @@ func TestAMPHop(t *testing.T) {
 // TestPayloadSize tests the payload size calculation that is provided by Hop
 // structs.
 func TestPayloadSize(t *testing.T) {
+	t.Parallel()
+
 	hops := []*Hop{
 		{
 			PubKeyBytes:      testPubKeyBytes,
@@ -181,7 +183,9 @@ func TestPayloadSize(t *testing.T) {
 			AmtToForward:     1200,
 			OutgoingTimeLock: 700000,
 			MPP:              record.NewMPP(500, [32]byte{}),
-			AMP:              record.NewAMP([32]byte{}, [32]byte{}, 8),
+			AMP: record.NewAMP(
+				[32]byte{}, [32]byte{}, 8,
+			),
 			CustomRecords: map[uint64][]byte{
 				100000:  {1, 2, 3},
 				1000000: {4, 5},
@@ -190,6 +194,69 @@ func TestPayloadSize(t *testing.T) {
 		},
 	}
 
+	blindedHops := []*Hop{
+		{
+			// Unblinded hop to introduction node.
+			PubKeyBytes:      testPubKeyBytes,
+			AmtToForward:     1000,
+			OutgoingTimeLock: 600000,
+			ChannelID:        3432483437438,
+			LegacyPayload:    true,
+		},
+		{
+			// Payload for an introduction node in a blinded route
+			// that has the blinding point provided in the onion
+			// payload, and encrypted data pointing it to the next
+			// node.
+			PubKeyBytes:   testPubKeyBytes,
+			EncryptedData: []byte{12, 13},
+			BlindingPoint: testPubKey,
+		},
+		{
+			// Payload for a forwarding node in a blinded route
+			// that has encrypted data provided in the onion
+			// payload, but no blinding point (it's provided in
+			// update_add_htlc).
+			PubKeyBytes:   testPubKeyBytes,
+			EncryptedData: []byte{12, 13},
+		},
+		{
+			// Final hop has encrypted data and other final hop
+			// fields like metadata.
+			PubKeyBytes:      testPubKeyBytes,
+			AmtToForward:     900,
+			OutgoingTimeLock: 50000,
+			Metadata:         []byte{10, 11},
+			EncryptedData:    []byte{12, 13},
+			TotalAmtMsat:     lnwire.MilliSatoshi(900),
+		},
+	}
+
+	testCases := []struct {
+		name string
+		hops []*Hop
+	}{
+		{
+			name: "clear route",
+			hops: hops,
+		},
+		{
+			name: "blinded route",
+			hops: blindedHops,
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Parallel()
+			testPayloadSize(t, testCase.hops)
+		})
+	}
+}
+
+func testPayloadSize(t *testing.T, hops []*Hop) {
 	rt := Route{
 		Hops: hops,
 	}