Remove usage of `helpers` method in CI config DSL

Problem

In the CI config DSL we found out that the use of helpers method was misused sometimes, causing confusion and potential bugs. It also makes the DSL unnecessarily complex and verbose.

Solution

Remove the use of helpers entirely and create them automatically when using entry method.

Example:

entry :variables, Entry::Variables,
  description: 'Environment variables available for this job.',
  inherit: false

will create also variables_defined? and variables_value methods.