Allow anonymous access to Epic Issues API for public groups & epics

Proposal

See documentation at https://docs.gitlab.com/ee/api/epic_issues.html

This endpoint is authenticated even though public groups and epics are available anonymously via the API.

For example try:

• https://gitlab.com/api/v4/groups/gitlab-com%2fgl-infra/epics/447
• https://gitlab.com/api/v4/groups/gitlab-com%2fgl-infra

versus:

• https://gitlab.com/api/v4/groups/gitlab-com%2fgl-infra/epics/447/issues

The endpoint should not be authenticated but should be available to anonymous users, showing the issues that are available via the user interface (gitlab-com/gl-infra&447 (closed)).