routerrpc: extend HTLC forward interceptor resp with modification fields

This commit extends the forward HTLC intercept response with fields that
can be used in conjunction with a `ResumeModified` action to modify the
intercepted HTLC p2p message.

intercepted_forward.go

@@ -54,7 +54,7 @@ func (f *interceptedForward) Resume() error {
 
 // ResumeModified notifies the intention to resume an existing hold forward with
 // a modified htlc.
-func (f *interceptedForward) ResumeModified(_ fn.Option[lnwire.MilliSatoshi],
+func (f *interceptedForward) ResumeModified(_, _ fn.Option[lnwire.MilliSatoshi],
 	_ fn.Option[lnwire.CustomRecords]) error {
 
 	return ErrCannotResume