Expose ExtractItems as the opposite of RemoveItems

This exposes a new method on TypedValue called ExtractItems. Whereas
RemoveItems takes a fieldpath.Set and returns the typed value with the
provided set items removed from it, ExtractItems returns ONLY those
items provided by the fieldpath.Set, effectively extracting them from
the original TypedValue.

internal/fixture/state.go

@@ -341,6 +341,83 @@ func (f ForceApplyObject) preprocess(parser Parser) (Operation, error) {
 	return f, nil
 }
 
+// ExtractApply is a type of operation. It simulates extracting an object
+// the state based on the manager you have applied with, merging the
+// apply object with that extracted object and reapplying that.
+type ExtractApply struct {
+	Manager    string
+	APIVersion fieldpath.APIVersion
+	Object     typed.YAMLObject
+}
+
+var _ Operation = &ExtractApply{}
+
+func (e ExtractApply) run(state *State) error {
+	p, err := e.preprocess(state.Parser)
+	if err != nil {
+		return err
+	}
+	return p.run(state)
+}
+
+func (e ExtractApply) preprocess(parser Parser) (Operation, error) {
+
+	tv, err := parser.Type(string(e.APIVersion)).FromYAML(FixTabsOrDie(e.Object))
+	if err != nil {
+		return nil, err
+	}
+	return ExtractApplyObject{
+		Manager:    e.Manager,
+		APIVersion: e.APIVersion,
+		Object:     tv,
+	}, nil
+}
+
+type ExtractApplyObject struct {
+	Manager    string
+	APIVersion fieldpath.APIVersion
+	Object     *typed.TypedValue
+}
+
+var _ Operation = &ExtractApplyObject{}
+
+func (e ExtractApplyObject) run(state *State) error {
+	if state.Live == nil {
+		return state.ApplyObject(e.Object, e.APIVersion, e.Manager, true)
+	}
+	// Get object from state and convert it to current APIVersion
+	current, err := state.Updater.Converter.Convert(state.Live, e.APIVersion)
+	if err != nil {
+		return err
+	}
+	// Get set based on the manager you've applied with
+	set := fieldpath.NewSet()
+	mgr := state.Managers[e.Manager]
+	if mgr != nil {
+		// we cannot extract a set that is for a different version
+		if mgr.APIVersion() != e.APIVersion {
+			return fmt.Errorf("existing managed fieldpath set APIVersion (%s) differs from desired (%s), unable to extract", mgr.APIVersion(), e.APIVersion)
+		}
+		// trying to extract the fieldSet directly will return everything
+		// under the first path in the set, so we must filter out all
+		// the non-leaf nodes from the fieldSet
+		set = mgr.Set().Leaves()
+	}
+	// ExtractFields from the state object based on the set
+	extracted := current.ExtractItems(set)
+	// Merge ApplyObject on top of the extracted object
+	obj, err := extracted.Merge(e.Object)
+	if err != nil {
+		return err
+	}
+	// Reapply that to the state
+	return state.ApplyObject(obj, e.APIVersion, e.Manager, true)
+}
+
+func (e ExtractApplyObject) preprocess(parser Parser) (Operation, error) {
+	return e, nil
+}
+
 // Update is a type of operation. It is a controller type of
 // update. Errors are passed along.
 type Update struct {