Use Sentry issue priority to specify `severity` on outgoing events

### Problem Statement

My team uses Sentry to create incidents on Pagerduty when a new event is added to an issue. Sometimes there are noisy issues that come from error logs or thrown exceptions, and we want to be able to control the noise without outright archiving the issue.

I set up Event Orchestration in Pagerduty that sets the urgency of the incident based on the `severity` sent by Sentry. When the severity is `error`, we page the on-call. When the severity is `warning`, we file an incident but do not page the oncall.

I'd like to be able to set a `severity` level based on the Sentry Issue priority, rather than the underlying event level. This way, if a noisy issue pages the on-call, they would be able to set the Issue Priority in Sentry to `Medium`. Then they will still receive incidents, but won't be woken up in the middle of the night from a page.


Sentry currently only allows me to filter based off of event level, not Issue Priority.


### Solution Brainstorm

I'd like to be able to set a filter based off of Issue Priority. This way, I could set up an alert that declares:

1. When an event is captured by Sentry, and the Issue's Priority is High
2. Send a notification to Pagerduty with Error severity

And another alert that could declare:

1. When an event is captured by Sentry, and the Issue's Priority is Medium
2. Send a notification to Pagerduty with Warning severity.

And so on for Low priority issues.

### Product Area

Alerts