Port "Extend API: Pipeline Schedule Variable" to EE

What does this MR do?

Port "Extend API: Pipeline Schedule Variable" to EE, due to ee-compat failure.

I just fixed

 > g di
diff --cc lib/api/entities.rb
index e3771acdd7,9cac2ff248..0000000000
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@@ -875,14 -818,7 +875,14 @@@ module AP

      class Variable < Grape::Entity
        expose :key, :value
-       expose :protected?, as: :protected
+       expose :protected?, as: :protected, if: -> (entity, _) { entity.respond_to?(:protected?) }
 +
 +      # EE
 +      expose :environment_scope, if: ->(variable, options) {
 +        if variable.respond_to?(:environment_scope)
 +          variable.project.feature_available?(:variable_environment_scope)
 +        end
 +      }
      end

      class Pipeline < PipelineBasic

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13653
• https://gitlab.com/gitlab-org/gitlab-ce/issues/34518