Option to customize NamespaceTransformer overwrite behaviour (#4708)

* Option to customize NamespaceTransformer overwrite behaviour

* Code review feedback

plugin/builtin/namespacetransformer/NamespaceTransformer.go

@@ -17,6 +17,7 @@ import (
 type plugin struct {
 	types.ObjectMeta `json:"metadata,omitempty" yaml:"metadata,omitempty" protobuf:"bytes,1,opt,name=metadata"`
 	FieldSpecs       []types.FieldSpec `json:"fieldSpecs,omitempty" yaml:"fieldSpecs,omitempty"`
+	UnsetOnly        bool              `json:"unsetOnly" yaml:"unsetOnly"`
 }
 
 //noinspection GoUnusedGlobalVariable
@@ -42,6 +43,7 @@ func (p *plugin) Transform(m resmap.ResMap) error {
 		if err := r.ApplyFilter(namespace.Filter{
 			Namespace: p.Namespace,
 			FsSlice:   p.FieldSpecs,
+			UnsetOnly: p.UnsetOnly,
 		}); err != nil {
 			return err
 		}