Introduce basic work item listing endpoints

What does this MR do and why?

Introduce basic work item listing REST endpoints:

1. /namespaces/:fullpath_or_id/-/work_items
2. /projects/:fullpath_or_id/-/work_items
3. /groups/:fullpath_or_id/-/work_items

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout the branch
2. Enable the feature flag

In the browser

You can directly put this in your browser URL:

• For a project with full path gitlab/test (note URL encoded /!): http://gdk.test:3000/api/v4/projects/gitlab%2Ftest/-/work_items
• For a group: http://gdk.test:3000/api/v4/group/gitlab/-/work_items
• For a namespace (group or project: http://gdk.test:3000/api/v4/group/gitlab/-/work_items

You can also add params for tile fields or features filtering:

• Labels, Milestones, Description: http://gdk.test:3000/api/v4/groups/gitlab/-/work_items?features=labels,milestone,description
• Fields: http://gdk.test:3000/api/v4/groups/gitlab/-/work_items?fields=state,confidential
• Combined: http://gdk.test:3000/api/v4/groups/gitlab/-/work_items?fields=state,confidential&features=labels,milestone,description

curl

curl --request GET \
  --url 'http://gdk.test:3000/api/v4/namespaces/gitlab/-/work_items?features=labels' \
  --header 'private-token: PAT'

MR acceptance checklist

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