Create PublisherData within NodeData

[Yadunund]

This PR replaces class rmw_publisher_data_t with class PublisherData which provides thread-safe access to publisher functions. An instance of PublisherData is created via the NodeData class. rmw_publisher->data is now set to rmw_node_t. The assumption here is that the node outlives the publisher similar to the assumption that the context outlives the node. From this rmw_node_t raw_ptr, we are able to obtain it's NodeData and then lookup the PublisherData for the publisher.

[Yadunund]

I had to rely on a reinterpret_cast<const_cast<>> to be able to store the const rmw_node_t * node available to rmw_create_publisher() within rmw_publisher->data as the data field is void * and not const void *.

This is a bit messy. Another alternative I considered was storing the rmw_context_impl_s * ptr here and then including an unordered_map in this class to obtain the NodeData for a given rmw_publisher_t but this involves extra bookkeeping that we need to do when we create and destroy a publisher. Open to other suggestions!