Create group issue boards API endpoint

Follow up of:

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2467/diffs

Description

Expose column lists, similar to https://docs.gitlab.com/ee/api/boards.html for project boards.

Out of scope

Saved config API is to be worked on separately here: https://gitlab.com/gitlab-org/gitlab-ee/issues/4266