Use work item IID for requirements filtering

@felipe_artur mentioned in #329435 (comment 1165768324):

Requirements IIDs are also still used on GraphQL API endpoints for filtering, but these are also going to be deprecated.

Before we can proceed with requirements API deprecation in #329432, we'll need to support work item IID for filtering as well as requirement IID to support customers moving across to use the work item IID.

• Modify the finder
• Add an argument for work_item_iid or similar in the GraphQL resolver
• Handle if the work item isn't a Requirement or if the Requirement isn't in the project, etc

The following query should work:

query {
  project(fullPath: "gitlab-org/gitlab") {
    requirements(work_item_iids: [1, 2]) {
      nodes {
        title
        iid
        work_item_iid
      }
    }
  }
}