Add last name argument to requestDisplayName

Right now the https://lensstudio.snapchat.com/api/classes/UserContextSystem/ user context system's request display name function just returns a string consisting of the first & last name.

Many people want to use only the first name so I suggest adding a second argument to this request, a boolean for including the last name that defaults to true if not defined. This allows an easy way to split it up without using the unreliable splitting at first space method & adding as an argument ensures backward compatibility and simplicity.