[Go] Updated the 'callback' and 'extend' examples to match the 'director' one.

After the documentation update on how to utilize the director feature with
commit @17b1c1c the 'callback' and 'extend' examples needed an update as well.

Examples/go/extend/ceo.go

@@ -0,0 +1,37 @@
+package example
+
+type CEO interface {
+	Manager
+	deleteManager()
+	IsCEO()
+}
+
+type ceo struct {
+	Manager
+}
+
+func (p *ceo) deleteManager() {
+	DeleteDirectorManager(p.Manager)
+}
+
+func (p *ceo) IsCEO() {}
+
+type overwrittenMethodsOnManager struct {
+	p Manager
+}
+
+func NewCEO(name string) CEO {
+	om := &overwrittenMethodsOnManager{}
+	p := NewDirectorManager(om, name)
+	om.p = p
+
+	return &ceo{Manager: p}
+}
+
+func DeleteCEO(p CEO) {
+	p.deleteManager()
+}
+
+func (p *ceo) GetPosition() string {
+	return "CEO"
+}