Allow pre-existing issues/tickets from 3rd party issue tracking integrations to change issue assignee

### Problem Statement

Currently, Issues/Tickets from external ticketing platforms (ex. - JIRA, GH, Linear) can only influence the Sentry Issue’s assignee if:
a) the ticket was generated alongside the Sentry Issue, and
b) re-assigning the ticket’s assignee post-link

### Solution Brainstorm

Example Scenario: Customer links an existing JIRA ticket to a new Sentry issue. The JIRA ticket’s assignee auto-populates within the Sentry Issue.

### Product Area

Settings - Integrations