refactor+htlcswitch: method for TLV payload parsing logic

In preparation for calling the TLV payload parsing logic recursively for
when we need to peel dummy hops from an onion, this commit creates a new
extractTLVPayload function. This is a pure refactor.

htlcswitch/hop/iterator_test.go

@@ -287,7 +287,6 @@ func TestDecryptAndValidateFwdInfo(t *testing.T) {
 					encryptedData: testCase.data,
 					blindingPoint: testCase.payloadBlinding,
 				}, false,
-				make(map[tlv.Type][]byte),
 			)
 			require.ErrorIs(t, err, testCase.expectedErr)
 		})