Allow Customization of Custom Details Section in PagerDuty Incidents and Alerts

_Note: This is just a copy of https://github.com/getsentry/getsentry/issues/12296, filed by @Prithvirajkumar. Because it was filed in the `getsentry` repo (which is private), it couldn't be transferred into `sentry` (which is public`), so instead it's copied over._ **Description:** Sentry customers would like the option to customize the Custom Details section within both the Incident page and the Alert page in PagerDuty. Essentially, they would like more control over the information presented in these sections to tailor it to their specific needs. **Context:**    **Acceptance Criteria:** - Implement functionality within the Sentry - PagerDuty integration settings to customize the content of the Custom Details section. - Ensure that changes made in the customization settings reflect in both the Incident page and the Alert page in PagerDuty.