LibCore: Convert CTCPSocket to ObjectPtr, add construct() helper

The C_OBJECT macro now also inserts a static construct(...) helper into
the class. Now we can make the constructor(s) private and instead call:

    auto socket = CTCPSocket::construct(arguments);

construct() returns an ObjectPtr<T>, which we'll later switch to being
a NonnullRefPtr<T>, once everything else in in place for ref-counting.

Libraries/LibCore/CHttpJob.h

@@ -26,7 +26,7 @@ private:
     };
 
     CHttpRequest m_request;
-    CTCPSocket* m_socket { nullptr };
+    ObjectPtr<CTCPSocket> m_socket;
     State m_state { State::InStatus };
     int m_code { -1 };
     HashMap<String, String> m_headers;