GitLab Issues API (and/or graphql) should include information on whether an Issue was closed as a duplicate

When an issue is closed as a duplicate (using /duplicate), the issue will show a Closed(Duplicate) status in the Web UI.

However, there is no way (as far as I'm able to tell through the documentation) to determine whether an Issue was closed as a duplicate from the API or GraphQL.

Having this available through the API would allow us to easily build some processes using our triage-ops tooling around this event.

How does it work?

When an issue is closed as a duplicate, we see

    "closed_as_duplicate_of": "http://gitlab.example.com/api/v4/projects/1/issues/76",

Otherwise, the issue is marked as:

    "closed_as_duplicate_of": null,

cc @jarv @steveazz