Unified push nextcloud notifications

[gavine99]

changes to allow any UnifiedPush distributor to deliver push messages in generic/f-droid build.

added a setting item to reset push notification registrations in case of issues with push (UnifiedPush distributor can sometimes get out of sync). this same setting item can also be used to reset firebase push in case of issues with it too.

a similar pr has been submitted for the nextcloud android talk app - nextcloud/talk-android#4169

[github-actions]

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[Fuseteam]

@gavine99 i notice the DCO is failing, if you click on details, you'll find details in what you need to do