Add Group secret variables

What does this MR do?

This MR adds a functionality for Group secret variables

EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2373

Are there points in the code the reviewer needs to double check?

• Variables of sub-groups will be included recursively.
• This MR doesn't affect a consistency for Project secret variables
• This MR doesn't add a functionality for Namespace

Why was this MR needed?

Issued at https://gitlab.com/gitlab-org/gitlab-ce/issues/12729. Frequently duplicated. More than 50 upvotes.

Screenshots (if relevant)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11822#note_31298896

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added -> This will be implemented in https://gitlab.com/gitlab-org/gitlab-ce/issues/34519
• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/12729
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11822
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11078
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11186
• https://gitlab.com/gitlab-org/gitlab-ce/issues/31799
• https://gitlab.com/gitlab-org/gitlab-ce/issues/24196 (Protect variables)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/12729