Summarizer notification channel

[peersky]

For a various reasons some of monitoring activity makes sense to report in Slack as aggregate summary over some time window. I.e statistical data: numbers of transactions, numbers of privileged account activity etc.

Hence It would be great if we have following workflow:

1. Any Monitors that must report to such summary dashboard are connected to "Summarizer" action.
2. Such "Summarizer" action takes alert information / metadata and records it in to KVS store (or any other DB solution we can get)
3. Another Autotask is invoked on schedule basis to read KVS store with reports and process them. (Idea: Add anomaly detector here!)
4. Send summary webhook sentinel that will post an update to slack/telegram/email etc

From user side ideally we just specify "SummaryWriter" in Notification Channel.