Opsgenie Integration: add notes to Opsgenie Sentry alerts

### Problem Statement

Hi team, one of our customers that uses Sentry with Opsgenie has requested the capability to add notes in the alerts. Their specific use case was to add a runbook link in the alert itself.

We currently offer this for our Slack integration (see screenshot), but not for our Opsgenie integration.


### Solution Brainstorm

Add a customizable "Notes" section for Sentry alerts with the Opsgenie integration

### Product Area

Settings - Integrations

┆Issue is synchronized with this [Jira Improvement](https://getsentry.atlassian.net/browse/FEEDBACK-2245) by [Unito](https://www.unito.io)