Issue feature availability supports namespace level issues

What does this MR do and why?

Now that issues can be created at the namespace level, feature availability checks cannot rely on the fact that an issue will always belong to a project

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer

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

3. Fetch assignee widget for the created group level work items

   {
      group(fullPath: "flightjs") {
        name
        workItems {
          nodes {
            widgets {
              ... on WorkItemWidgetAssignees {
                allowsMultipleAssignees
                canInviteMembers
              }
            }
          }
        }
      }
    }

4. The query above should fail before the change in this MR

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 #422960 (closed)