Allow separate notification grace periods for each individual “group by” attribute

Initial discussion [here](https://community.graylog.org/t/grace-period-custom-fields-key-with-events/26686/2)

Use case:
I have a simple event that triggers an alert when any host generates over 2000 messages per hour. The aggregation is done on the host source name with a count.

I want to add a grace period to stop any one host spamming emails after triggering. But I still want to receive alerts from other hosts that trigger the event.

Currently the grace period applies to the entire event. Setting it disables alerts from all hosts. It would be great if grace periods could apply to each individual "group by" attribute.

(Issue experienced on Graylog 4.2.13-1 on Centos 7.9)