Add civetweb user-connection-data bindings to Http.alusus

[heshammahamed]

Why

WebPlatform currently tracks the mapping between a raw connection
pointer and its WsConnection object (the object handed to the user
for each WebSocket connection) using a shared Map. Civetweb runs each
connection's callbacks on that connection's own dedicated thread for
its entire lifetime, so different connections' ready/data/close
handlers run concurrently on different threads — meaning this shared
Map is read and written from multiple threads at once, which make a race condition.

What this PR does

Adds two new bindings in Http.alusus:

• setUserConnectionData → mg_set_user_connection_data
• getUserConnectionData → mg_get_user_connection_data

These let civetweb store a per-connection pointer directly on the
connection object itself. Since each connection is handled by its own
dedicated thread for its whole life, this per-connection data is only
ever touched by that one thread — so no lock needed.

This will let us map each connection directly to its WsConnection
without a shared Map at all.

References

• https://github.com/civetweb/civetweb/blob/master/docs/api/mg_set_user_connection_data.md
• https://github.com/civetweb/civetweb/blob/master/docs/api/mg_get_user_connection_data.md

[sarmadka]

هل يمكن تغيير هذه إلى حدد_بيانات_إتصال_المستخدم، أي استخدام المصطلح "حدد" بدل "عين" كترجمة لset كي تكون متوافقة مع الترجمات في المكتبات الأخرى؟
عموما نحن نترجم "حدد" إلى set و "عين" إلى assign.