[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_trickle_timer.cpp

@@ -30,9 +30,9 @@
 
 #include "common/code_utils.hpp"
 #include "common/debug.hpp"
-#include "common/instance.hpp"
 #include "common/num_utils.hpp"
 #include "common/trickle_timer.hpp"
+#include "instance/instance.hpp"
 
 static ot::Instance *sInstance;