[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_pool.cpp

@@ -30,8 +30,8 @@
 
 #include <openthread/config.h>
 
-#include "common/instance.hpp"
 #include "common/pool.hpp"
+#include "instance/instance.hpp"
 
 #include "test_util.h"