Allow enriching alerts with issue context

### Problem Statement

Sentry allows settings up alerts to automatically notify issue owners about the issues. The raised alerts are automatically enriched with tags and various other data. It is then possible to use this data on receiver end, for example in OpsGenie, to properly define priority and severity of the issue.

The problem is that we can only add data from the context of the even, which triggered it. We can't add issue/alert context. And things like severity, priority or escalation policy usually depend on it.

For instance, we can't enrich alert payload with information about the number of effected users, or events per minute, etc.

### Solution Brainstorm

The solution would be to allow enriching alert payload for **all** notification channels with the alert context. There may be a set of default fields, like event frequency or the total number of effected users, but it'll be also useful to be able to define custom alert metrics to propagate as tags in alert payload.

### Product Area

Alerts