Delegate CiVariable fabrication to API

What does this MR do?

Instead of Resource::CiVariable fabricating via the browser_ui, let's delegate to the API instead. (will shave off roughly 3-4s every use)

resource_web_url - not all POST requests will return a { web_url } back from the JSON body. In these cases - persist. don't fail with an error

rel: https://gitlab.com/gitlab-org/gitlab-ce/issues/57562

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

This MR will increase testing performance by 3-4s per test that uses the Resource::CiVariable.fabricate! / Resource::CiVariable.fabricate_via_api! method