Modernizing the codebase

[mickeyl]

I wonder whether it would be feasible to update the codebase a bit with regards to modern C++ constructs.

In particular, esp-nimble-cpp relies a lot on pointers (e.g., for callbacks). Devices such as std::unique_ptr and std::shared_ptr could improve this a lot.

But also trivial things like using auto and for (auto& item: collection) could help to make the codebase more simple to read, more friendly towards contributions, and better maintainable.

What's your take on that?