Small Feature request

Remove asymmetry in subscribing/publishing to serialized message

Current behavior

I can subscribe to a serialized message directly by calling create_subscription with a callback of the form

[](std::shared_ptr<rcutils_char_array_t> msg) { ... }

However, I call publish handing in a raw pointer:

Improved behavior

I can call publish also by handing in a shared_ptr of a serialized message.