Alerts about crash-free sessions from specific error

### Problem Statement

The traffic on our site is very intermittent. We buy ads, do A/B testing, then stop buying ads so the amount of traffic decreases. And we want to be notified when >N% of users encounter certain errors that we manually send to Sentry.

There are several options of alerts we can set up:
* Number of errors - when there are >N errors or +N% of errors relative to the moment in the past
* Number of users experiencing errors - when there are >N users experiencing errors or +N% of users experiencing errors relative to the moment in the past.
* Crash free session rate / Crash free user rate - if >N% of users experience errors

The first two do not look suitable for our case, we cannot rely on a specific number of errors, and we cannot compare the number of errors with the moment in the past. But in these options, we can filter errors in any way we like.

The third one looks perfect, but we could only filter error by "release version", not by the message or any other filterable property.

### Solution Brainstorm

* Add ability to filter errors by filterable properties for the "crash-free session rate" alerts.

### Product Area

Alerts