Implement UserAchievementIconFetched_t callback

[LuisMayo]

Currently the UserAchievementIconFetched_t callback cannot be handled.

This is a callback that runs when the achievement icon has been retrieved from the server. This callback is necessary because get_achievement_icon may not return the icon if Steam hasn't downloaded it from the server yet. So with this callback we should be able to wait for it until Steam finally has the data.

This is also how get_achievement_icon is supposed to work according to Steamworks Docs.
Implementing the function call without the callback itself (current situation) is just tossing a coin on whether the call will be successful or not.

Again, thanks a lot for this library <3