XEP-0357 (a.k.a Push Notifications) Support

[vsevex]

Current Limitations and Proposed Solution:

Whixp currenly leverages the Publish-Subscribe (PubSub) mechanism, which is undoubtly powerful (from my aspect, ofc). However, for mobile applications, push notifications plays pivotal role for delivering real-time updates and enhancing user engagement. While XMPP offers the XEP-0357 extension for push notifications, implementing it to work with Firebase Cloud Messaging within the XMPP server can be painful.

This issue proposes a proxy server to bridge the app the gap between the Whixp client (user's server) and the Whixp component (app server). The intermediary would streamline the integration of FCM, simplifying the implementation of push notifications within the mobile app (targeting both Android and iOS environments).