[app] Add UnifiedPush for github build flavours as alternative for FCM

[wax911]

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https://github.com/AniTrend/anitrend-v2/issues?q=is%3Aissue+is%3Aclosed

Feature Information

Explore UnifiedPush: a decentralized, open-source push notification protocol, to use since our github/f-droid flavours are stripped of any google sdks including firebase.

Solution Information

See the documentation here

Additional Context

• Add com.github.UnifiedPush:android-connector to libs.versions.toml and add it as a githubImplementation in app/build.gradle.kts
• Make approprieate changes to the github version of MessagingService.kt using the developer the UnifiedPush developer guide

See: UnifiedPush for more dev related details