Add group level work item description diff endpoints

What does this MR do and why?

Adding two new routes to our internal REST API. This are necessary in the new work items API to fetch legacy diffs. In the future they will be replaced in the GQL API. Created &12091 to track that work

How to set up and validate locally

1. Go to GraphiQL locally at http://127.0.0.1:3000/-/graphql-explorer
2. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
3. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer so you can use it in the next query

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
          iid
        }
      }
    }

4. update the description on the newly created work item

   mutation {
      workItemUpdate(input: {id: "<gid_from_previous_step>", descriptionWidget: {description: "updated description"}}) {
        errors
        workItem {
          iid
          workItemType {
            name
          }
          widgets {
            ... on WorkItemWidgetDescription {
              description
            }
          }
        }
      }
    }

5. Fetch description diffs:

    {
      group(fullPath: "flightjs") {
        id
        workItem(iid: "<work_item_iid>") {
          nodes {
            widgets {
              ... on WorkItemWidgetNotes {
                discussions {
                  nodes {
                    notes {
                      nodes {
                        systemNoteMetadata {
                          descriptionVersion {
                            diffPath
                            deletePath
                          }
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }

   The query would error before this change. Now diffPath will be returned and you can use it in the url to get the diff. This is a new route added in this MR too

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #431215 (closed)