Allow creating project-level work items epics

What does this MR do and why?

Related to #516896 (closed)

This MR introduces the missing permissions that allow us to create project-level work items with the type Epic.

Creating and listing this type of work item will be guarded by the feature flag project_work_item_epics, and in the same way as group-level epics, it requires the epics licensed feature.

These changes only affect the graphQL API endpoints, we don't have access to creating this type via the UI yet.

To note: Given that we can use the ProjectType.issues or ProjectType.work_items endpoints for listing work items, we have to prevent read_issue and read_work_item abilities when the item has type Epic and license is not available (or FF is disabled).

References

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• Stage I: Create and List Project Level Epics (&16668)

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example queries

Feature Flag enabled

create project-level epic	list project-level epics

Feature Flag disabled

create project-level epic	list project-level epics

How to set up and validate locally

1. In rails console enable feature flag

   Feature.enable(:project_work_item_epics)

2. Visit GraphQL explorer http://gdk.test:3000/-/graphql-explorer and very that the following queries create and fetch a project-level work item with type Epic:

graphQL queries

mutation createEpicWorkItem {
  workItemCreate(input: {
    namespacePath: "GROUP_PATH/PROJECT_PATH", title: "Project Epic",
    workItemTypeId: "gid://gitlab/WorkItems::Type/8"
  }) {	
    workItem {
      title
      webUrl
      workItemType {
        name
      }
    }
    errors
  }
}

query getProjectWorkItems {
  project(fullPath: "GROUP_PATH/PROJECT_PATH") {
    workItems(types: [EPIC]) {
      edges {
        node {
          title
          webUrl
          workItemType { name }
        }
      }
    }
  }
}

# Because work items are an issue record, they can be fetched using the Issues query too
query getProjectIssues {
  project(fullPath: "GROUP_PATH/PROJECT_PATH") {
    issues(types: [EPIC]) {
      edges {
        node {
          title
        }
      }
    }
  }
}

3. Disable the feature flag and verify that the mutation returns an error and the queries are empty