Add TrackableFilter interface (#4410)

* add kio filter interface

This interface is an extension of the Filter interface which can be used
for filters which are capable of tracking which fields they mutate.

* add TrackableSetter struct to filtersutil

This struct provides an abstraction to help Filters implement the
TrackableFilter interface

* implement TrackableFilter with annotations

This updates the annotations filter to implement the TrackableFilter
interface by reusing the TrackableSetter abstraction provided by
filtersutil.

This is done to provide a generic and consistent experience across the
filters

* implement TrackableFilter with labels

This updates the labels filter to implement the TrackableFilter
interface by reusing the TrackableSetter abstraction provided by
filtersutil.

This is done to provide a generic and consistent experience across the
filters

api/filters/filtersutil/setters.go

@@ -31,3 +31,39 @@ func SetEntry(key, value, tag string) SetFn {
 		})
 	}
 }
+
+type TrackableSetter struct {
+	// SetValueCallback will be invoked each time a field is set
+	setValueCallback func(key, value, tag string, node *yaml.RNode)
+}
+
+// WithMutationTracker registers a callback which will be invoked each time a field is mutated
+func (s *TrackableSetter) WithMutationTracker(callback func(key, value, tag string, node *yaml.RNode)) {
+	s.setValueCallback = callback
+}
+
+// SetScalar returns a SetFn to set a scalar value
+// if a mutation tracker has been registered, the tracker will be invoked each
+// time a scalar is set
+func (s TrackableSetter) SetScalar(value string) SetFn {
+	origSetScalar := SetScalar(value)
+	return func(node *yaml.RNode) error {
+		if s.setValueCallback != nil {
+			s.setValueCallback("", value, "", node)
+		}
+		return origSetScalar(node)
+	}
+}
+
+// SetEntry returns a SetFn to set an entry in a map
+// if a mutation tracker has been registered, the tracker will be invoked each
+// time an entry is set
+func (s TrackableSetter) SetEntry(key, value, tag string) SetFn {
+	origSetEntry := SetEntry(key, value, tag)
+	return func(node *yaml.RNode) error {
+		if s.setValueCallback != nil {
+			s.setValueCallback(key, value, tag, node)
+		}
+		return origSetEntry(node)
+	}
+}