multi: add IsBlinded to lnrpc.Invoice for nicer UX

The BlindedPathConfig struct is nice for invoice creation but when we
use the Invoice message for viewing an invoice, it would be nicer to see
an "is_blinded" field.

invoices/invoices.go

@@ -489,6 +489,15 @@ func (i *Invoice) IsAMP() bool {
 	)
 }
 
+// IsBlinded returns true if the invoice contains blinded paths.
+func (i *Invoice) IsBlinded() bool {
+	if i.Terms.Features == nil {
+		return false
+	}
+
+	return i.Terms.Features.IsSet(lnwire.Bolt11BlindedPathsRequired)
+}
+
 // HtlcState defines the states an htlc paying to an invoice can be in.
 type HtlcState uint8