Add work items resolver

What does this MR do and why?

Exposes work items for a project, very similar to listing issues for a project. Because this is needed for #362860 (closed) it adds only bare minimum filtering (searching by title and type and sorting by title).

Related to #363299 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the feature

Feature.enable(:work_items)

2. Test with graphql query like this:

query {
  project(fullPath: "flightjs/flight") {
    workItems(types: ISSUE, state: opened, search: "asd", first: 5, sort: TITLE_ASC) {
      nodes {
        id
        title
      }
    }
  }
}

MR acceptance checklist

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