HTTPClient callback has no way of accessing non-global data

typically with callbacks a void* is passed when setting the callback, and then when something calls the callback, they will pass it that void*. this allows users to cast the void* in to whatever data they need within the callback.

without this, only static/global data is accessible from within the callback, which is quite limiting.
alternatively, something like std::function or a templated function could be used instead, to allow us to pass a lambda for the callback and capture state.

Inizia individuando i percorsi di registrazione e invocazione del callback di HTTPClient nell’SDK C++; l’issue non indica file o test specifici. Confronta l’approccio richiesto al contesto void* con il callback std::function o templated suggerito, quindi definisci il comportamento del callback supportato e aggiungi la copertura prima di considerare concluso il lavoro.