[core] define core/instance folder for instance modules (#9561)

This commit moves the `instance` module to a newly added folder
`core/instance` (from `core/common`.  Header file `extension.hpp`
and its example is also moved to the same folder.

tests/unit/test_child_table.cpp

@@ -33,7 +33,7 @@
 #include "test_util.h"
 #include "common/array.hpp"
 #include "common/code_utils.hpp"
-#include "common/instance.hpp"
+#include "instance/instance.hpp"
 #include "thread/child_table.hpp"
 
 namespace ot {