[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_srp_server.cpp

@@ -38,9 +38,9 @@
 
 #include "common/arg_macros.hpp"
 #include "common/array.hpp"
-#include "common/instance.hpp"
 #include "common/string.hpp"
 #include "common/time.hpp"
+#include "instance/instance.hpp"
 
 #if OPENTHREAD_CONFIG_SRP_SERVER_ENABLE && OPENTHREAD_CONFIG_SRP_CLIENT_ENABLE && \
     !OPENTHREAD_CONFIG_TIME_SYNC_ENABLE && !OPENTHREAD_PLATFORM_POSIX