Customize body of Pager Duty alert to send additional data

### Problem Statement

Currently, the Pager Duty integration allows you to specify the account and service to which to send an alert action. However, I can't add a short description or notes to what the alert assignee sees on PD. It’s mainly helpful for applicative errors where the error was thrown for a good reason and the fix is not in the code

### Solution Brainstorm

Add a free-form field when selecting a PD integration as the alert action where I am able to specify a description that is going to be sent with the payload of the alert to my PD instance.

### Product Area

Settings - Integrations

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-1979) by [Unito](https://www.unito.io)