Add support for querying work items by references

Related to #427594 (closed)

What does this MR do and why?

To support using references in the linked items widget input we need to add support for querying work items by reference.

This query requires a context project and a list of references that can be:

• URLs: The full URL for the work item, for example "https://gitlab.com/gitlab-org/gitlab/-/work_items/427594"
• Short references:
  • A hash followed by the work item IID for items within the context project (e.g. #1)
  • A project's full path followed by the IID for items that don't belong to the context project (e.g. gitlab-org/plan-stage/okr-sandbox#674).

Note: We limit the number of items that can be queried at the time because rendering references is an expensive operation. This limit matches the maximum number of items that can be included in the work items input (10).

Note 2: The query doesn't support work items at the group level (either with a group context or a group-level reference) because these references are not supported yet. Regardless, group-level work items are still behind the FF namespace_level_work_items, disabled by default.

Screenshots or screen recordings

How to set up and validate locally

Visit http://127.0.0.1:3000/-/graphql-explorer and test the following query using existing references

query getWorkItems {
  workItemsByReference(contextNamespacePath: "group-a/project-a", refs: ["#1", "group-a/project-a-2#4", "http://127.0.0.1:3000/public-group/public-project/-/work_items/6"]) {
    edges {
      node {
        id
        title
      }
    }
  }
}

example response

{
  "data": {
    "workItemsByReference": {
      "edges": [
        {
          "node": {
            "id": "gid://gitlab/WorkItem/405",
            "title": "Test Task"
          }
        },
        {
          "node": {
            "id": "gid://gitlab/WorkItem/344",
            "title": "Another Test Task"
          }
        },
        {
          "node": {
            "id": "gid://gitlab/WorkItem/410",
            "title": "Test Objective"
          }
        }
      ]
    }
  }
}

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