API: Use Grape DSL to document access requests and members endpoints (!6269) · Merge requests · GitLab.org / GitLab FOSS · GitLab

Part of #21979 (closed), depends on gitlab-org/gitlab-ce!6267 and gitlab-org/gitlab-ce!6266.

Does this MR meet the acceptance criteria?

API support added
Tests
- Added for this feature/bug
- All builds are passing
Conform by the merge request performance guides
Conform by the style guides
Branch has no merge conflicts with master (if you do - rebase it please)
Squashed related commits together