Unified push nextcloud notifications

[gavine99]

changes to allow a 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

this pr is related to, but not dependent on, 3 other pr's;
nextcloud server - nextcloud/server#47763 (to provide enhanced ability for multiple notification handlers)
nextcloud notifications app - nextcloud/notifications#2027 (to provide enhanced ability for multiple notification handlers)
3rd party unified push app - https://codeberg.org/NextPush/uppush/pulls/17 (to allow existing 3rd party nextcloud unifiedpush app to send nextcloud notifications via unifedpush)

altogether, the prs above provide an end-to-end solution for nextcloud to deliver push notifications via UnifiedPush.

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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