lnwallet: add IsAdd helper to AuxHtlcDescriptor

We also add the IsAdd helper to the AuxHtlcDescriptor, as external
software using the aux framework might want to know which type of HTLC
this is.

lnwallet/aux_signer.go

@@ -116,6 +116,18 @@ func (a *AuxHtlcDescriptor) AddHeight(
 	return a.addCommitHeightLocal
 }
 
+// IsAdd checks if the entry type of the Aux HTLC Descriptor is an add type.
+func (a *AuxHtlcDescriptor) IsAdd() bool {
+	switch a.EntryType {
+	case Add:
+		fallthrough
+	case NoOpAdd:
+		return true
+	default:
+		return false
+	}
+}
+
 // RemoveHeight returns the height at which the HTLC was removed from the
 // commitment chain. The height is returned based on the chain the HTLC is being
 // removed from (local or remote chain).