Allow "control" and "candidate" function names

What does this MR do and why?

In GLEX use and try methods are aliased as control and candidate, respectively. This allows as to write:

experiment(:world_domination, ...) do |e|
   e.control {} # instead of e.use
   e.candidate {} # instead of e.try
end

We also have an experiment helper in the frontend but we can only use and try.

This MR, updates that utility function to also allow naming experiment code paths as control (like use) and candidate (like try) so we can write:

experiment('pipeline_editor_walkthrough', {
  control: () => { ... },
  candidate: () => { ... },
});

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