Draft: Add Push Notification with Play Service on generic flavor (full FLOSS)

[p1gp1g]

This PR is based on #5883, it is marked as draft until the previous PR is merged

It adds a component, FCM embedded UnifiedPush distributor, that allows to get push notifications with the Play Services, without using firebase-messaging, and the proprietary libs. It is therefore compatible with F-Droid inclusion.

It gives Play Services notifications with web push requests: so the notification proxy, isn't used

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[mahibi]

Thank you so much @p1gp1g i'm really looking forward to have UnifiedPush implemented 👍 👍 👍
We will take a close look as soon as time allows, it might not be this week though.

[mahibi]

i won't find the time this week. It's on my list though

[mahibi]

It's not forgotten. Will come back

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