Support filtering Alerts based on issue severity

### Problem Statement

I would like to generate Alerts whenever there are new events for High priority Issues. This is inline with the suggested [Alerts Best Practices](https://docs.sentry.io/product/alerts/best-practices/#reducing-noise).
```
Prioritize high priority issues: If you're getting too many notifications about non-error or low priority issues, add the 'high priority' condition to your alert configuration. That way, you'll only get alerts for high-priority issues.
```

Today's alerting functionality only supports filtering on priority for _new_ issues

### Solution Brainstorm

_No response_

### Product Area

Alerts