WIP: Implement variable presenter

What does this MR do?

This MR adds a facility for generating variables for runner. Mainly it consists of those functions.

• Variable Presenter (Delegated from each model)
• Variable Serializer (Construct a hash for runner)

This was suggested by @ayufan at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11771

Current relations

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33042 (closed)