Support receiving notifications via a long connection when running in the background to make it usable on devices that do not support GMS.

[NCPSNetworks]

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Summary

Mattermost Mobile cannot receive push notifications in regions where FCM is blocked or on devices without GMS installed.

Environment Information

• Device Name: iPhone 13 Pro Max
• OS Version: 17.5.1
• Mattermost App Version: 2.16.0
• Mattermost Server Version: 9.8.1

Expected behavior

I noticed that Mattermost Mobile cannot receive push notifications in regions where FCM is blocked or on devices without GMS installed.

Possible fixes

By establishing a long connection directly from the client to the Mattermost server, bypass FCM to receive push notifications (the client needs to run in the background).
It can be provided as an optional feature for users, disabled by default. (The benefit is that it supports these devices without requiring many changes, though it does consume slightly more power.)