No way to tell if an Issue was unlinked through Sentry

### Problem Statement

The Linear integration doesn't know if an issue has been unlinked on the Sentry side.

When you create or link a Sentry issue to a Linear issue we record this link on our end. But if you then remove the link/connection in Sentry (click the little X on an issue page), we have no way of knowing this happened and will continue to react to webhooks regarding that issue.

### Solution Brainstorm

We would like either (or both):

1. A webhook if an issue is unlinked in Sentry.
2. When looking up an issue in Sentry through API, get information on external issues linked to the issue. This way we could at least double check, before making changes based on webhooks, that the issue is still linked.

### Product Area

APIs