Add support for GetAuthTicketForWebApi

[rnels]

Motivation

Currently, it isn't possible to authenticate a steam user with a backend web server. Adding the function GetAuthTicketForWebApi and its associated callback handler GetTicketForWebApiResponse_t allows us to do so.

Changes

• Added init_user and shutdown_user to core.cpp
  • Seems like these were omitted by mistake, preventing the user.cpp callback handlers from registering
• Added user.getAuthTicketForWebApi method and OnTicketForWebApiResponse callback handler
  • Steamworks docs for GetAuthTicketForWebApi
  • Steamworks docs for GetTicketForWebApiResponse_t

This is my first time writing C++, so please let me know if you have any suggestions for improving my code. Also let me know if there are any additional requirements for contribution I've missed.