Userland: Split IPC endpoints into proxies and stubs

This enables support for automatically generating client methods.
With this added the user gets code completion support for all
IPC methods which are available on a connection object.

Userland/Libraries/LibIPC/Endpoint.h

@@ -1,36 +0,0 @@
-/*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#pragma once
-
-#include <AK/OwnPtr.h>
-#include <AK/String.h>
-
-namespace AK {
-class BufferStream;
-}
-
-namespace IPC {
-
-class Message;
-class MessageBuffer;
-
-class Endpoint {
-public:
-    virtual ~Endpoint();
-
-    virtual u32 magic() const = 0;
-    virtual String name() const = 0;
-    virtual OwnPtr<MessageBuffer> handle(const Message&) = 0;
-
-protected:
-    Endpoint();
-
-private:
-    String m_name;
-};
-
-}