API for CI Lint

## What does this MR do?
Add API for CI Lint. Can check if `.gitlab-ci.yml` content is valid.

## What are the relevant issue numbers?
Closes #15571 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] API support added
- [x] Documentation
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

cc @ubudzisz @grzesiek @yorickpeterse

See merge request !5953