[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_checksum.cpp

@@ -27,9 +27,9 @@
  */
 
 #include "common/encoding.hpp"
-#include "common/instance.hpp"
 #include "common/message.hpp"
 #include "common/random.hpp"
+#include "instance/instance.hpp"
 #include "net/checksum.hpp"
 #include "net/icmp6.hpp"
 #include "net/ip4_types.hpp"