Allow finding tombstones by issue group id

### Problem Statement

I am unable to associate tombstones back to the issue once delete and discarded. I need a way to find tombstones by its old issue group id, so that I can remove delete and discard issues correctly through the API.

### Solution Brainstorm

We should expose an endpoint that supports finding tombstones by its previous issue group id: `https://sentry.io/api/0/projects/{org-slug}/{project}/tombstones/?issue_group_id=12345`

### Product Area

Issues

Start by locating the API entry point for the tombstones endpoint and the existing handling of issue_group_id. Trace how discarded issues are represented, then verify the endpoint can find tombstones using their previous issue group ID and supports the stated delete and discard API workflow.

Written by the indexing model from the issue text.