Include group-level work items in group autocomplete sources

What does this MR do and why?

When visiting a work item at the group level, the autocomplete dialogue triggered by typing # in a comment or description will display work items at the group level.

To achieve this we modify Groups::AutocompleteService#issues to search for work items instead of issues by using WorkItems::WorkItemsFinder.

We also introduce the use of caching for the endpoints groups/autocomplete_sources#issues and projects/autocomplete_sources#issues that will expire every 3 minutes. To enable these changes both namespace_level_work_items and cache_autocomplete_sources_issues flags should be enabled.

Old query

IssuesFinder.new(current_user, { group_id: group.id, state: 'opened', include_subgroups: true })
            .execute.preload(project: :namespace)
            .with_work_item_type
            .select(:iid, :title, :project_id, :namespace_id, 'work_item_types.icon_name')

• small group: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29659/commands/92059
• large group: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29659/commands/92071

New query

WorkItems::WorkItemsFinder.new(current_user, { group_id: group.id, state: 'opened', include_descendants: true })
                          .execute.preload(project: :namespace)
                          .with_work_item_type
                          .select(:iid, :title, :project_id, :namespace_id, 'work_item_types.icon_name')

• small group: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29659/commands/92076
• large group: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29659/commands/92077

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Before	After

How to set up and validate locally

1. In rails console enable feature flags

   Feature.enable(:namespace_level_work_items)
   Feature.enable(:cache_autocomplete_sources_issues)

2. Visit any group and create two epics Epic A and Epic B. In a subgroup create another epic Epic C.
3. Also create several issues in a project belonging to the same group.
4. Visit the work item view of the epic e.g http://127.0.0.1:3000/groups/group-a/-/work_items/1 and type # in the comment box.
5. Verify that the results include the work items epics displaying the correct icon and the subsequent calls are served from cache.