[feat][struct_pack] not support shared_ptr

I saw the struct_pack library supports many STL containers
but after testing, I regretted that this library does not support the smart point of C++ 11.
like shared_ptr etc.

for example, I have a tree node struct like:

enum class type_e {
root = 0,
users = 1,
projs = 2,
};

struct Node {
type_e type;
std::string name;
std::string value;
std::list>> children;
};

for now, this library cannot serialize/deserialize the above Node structure.
you can reference another nonintrusive library named alpaca, which supports smart point.

Start by reading struct_pack's existing STL-container serialization support and compare its behavior with the shared_ptr example in the issue. Define how shared_ptr ownership, recursive Node values, deserialization, and possible cycles should work; the issue does not name a file or test, so completion criteria need to be established before implementation.

Written by the indexing model from the issue text.