Allow controls on Incident Key for Pagerduty

### Problem Statement

Feedback on Slack:

> The incident_key value [here](https://github.com/getsentry/sentry/blob/master/src/sentry_plugins/pagerduty/client.py#L41) is currently set to the group ID of whichever group triggered the alert. However Pagerduty uses that value to dedupe incidents, which means a single alert can trigger many pagerduty incidents.
> This [sentry alert](https://sentry.sentry.io/alerts/rules/sentry/12516560/) triggered [many](https://sentry.pagerduty.com/alerts/Q196N6ZTIOAX8E) [different](https://sentry.pagerduty.com/incidents/Q16B744SJG3JDH) [Pagerduty](https://sentry.pagerduty.com/alerts/Q0MUJPN4OIO3UA) [alerts](https://sentry.pagerduty.com/incidents/Q1D5AWTXAAHGMV). I’m wondering: could we change the incident_key value to be the ID of the alert rule instead of the group ID? Then the Pagerduty alerts would line up more with the alerts configured in Sentry.

### Solution Brainstorm

_No response_

### Product Area

Settings - Integrations