feature/set: add SetInvoiceAmp

AMP invoices need to signal:
 - AMPRequired in order to avoid being paid by older clients that don't
   support it.
 - Can't advertised MPP optional, otherwise older clients will attempt
   to pay the invoice with regular MPP payment.

Hence, the features advertised on AMP invoices are mutually exclusive
from those advertised on MPP. Create a new set to classify the two.

feature/set.go

@@ -22,6 +22,10 @@ const (
 	// SetInvoice identifies features that should be advertised on invoices
 	// generated by the daemon.
 	SetInvoice
+
+	// SetInvoiceAmp identifies the features that should be advertised on
+	// AMP invoices generated by the daemon.
+	SetInvoiceAmp
 )
 
 // String returns a human-readable description of a Set.
@@ -35,6 +39,8 @@ func (s Set) String() string {
 		return "SetNodeAnn"
 	case SetInvoice:
 		return "SetInvoice"
+	case SetInvoiceAmp:
+		return "SetInvoiceAmp"
 	default:
 		return "SetUnknown"
 	}