Dispatch an event to signal interest in defering the notifications

[nickvergessen]

Backportable hack for the biggest issue of #3743

This can be backported easily and when the notifications app is running nextcloud/notifications#651 it will help making everything faster

[nickvergessen]

/backport to stable19

[nickvergessen]

/backport to stable18

[nickvergessen]

To test:

1. Also apply the notifications app PR: [stable19] Allow to group push notifications via an event notifications#651
2. On top of that apply this in the notifications app https://gist.github.com/nickvergessen/8f53db1960b4d4933072b920edcbb74f
3. Create a conversation with multiple users (the more that have a push notification device assigned the better)
4. Start a call in that conversation
5. Check the log file /var/www/data/notifications.log from 2.

Before there was one request to the push proxy per user, now it's 1 per proxy, so should be 1 for everyone. This should be noticeable when comparing the duration

[danxuliu]

Tested (with a dummy push proxy, not a real one, but it should not make any difference) and works 👍