Add related merge requests to work items development widget

We need to be able to fetch merge requests where the work item has been mentioned. We already have this in the issues API, we should replicate in the work item widget interface of the GQL API

The related merge requests will return all the merge requests and we should be deduplicating the closing merge requests from the list on frontend i.e union of the relatedMergeRequests and closingMergeRequests by id