BE: Expose blocked and blocking work items for IssueType

Summary

As we are updating relationship icons across epic and issue boards and lists, we need to expose the information about issuables any given item blocks, so blockingIssuables and blockingEpics. Currently we are only exposing information about issuable that block any given item, which was implemented in !54152 (merged) and in #355739 (closed) a while ago.

Now we need the information about issuables and epics that an item is blocking, so frontend can consume the information needed and display:

Proposal

Step 1. Add a new field to IssueType called blocked_by_work_items that returns a list of work items that are blocking the issue

Example query

query itemsBlockingIssue {
  project(fullPath: "flightjs/Flight") {
    issue(iid: "42") {
      title
      blockedByWorkItems {
        edges {
          node {
            title
          }
        }
      }
    }
  }
}

Step 2. Add a new field to IssueType called work_items_blocked_by_issue that returns a list of work items that are blocked by the issue

Example query

query itemsBlockedByIssue {
  project(fullPath: "flightjs/Flight") {
    issue(iid: "43") {
      title
      workItemsBlockedByIssue {
        edges {
          node {
            title
          }
        }
      }
    }
  }
}