Unify variables serializers

Extract common elements from variables serializers into a new
base class.

app/serializers/ci/instance_variable_entity.rb → app/serializers/ci/basic_variable_entity.rb

 # frozen_string_literal: true
 
 module Ci
-  class InstanceVariableEntity < Grape::Entity
+  class BasicVariableEntity < Grape::Entity
     expose :id
     expose :key
     expose :value

app/serializers/ci/instance_variable_serializer.rb

@@ -2,6 +2,6 @@
 
 module Ci
   class InstanceVariableSerializer < BaseSerializer
-    entity InstanceVariableEntity
+    entity BasicVariableEntity
   end
 end

app/serializers/group_variable_entity.rb

 # frozen_string_literal: true
 
-class GroupVariableEntity < Grape::Entity
-  expose :id
-  expose :key
-  expose :value
-  expose :variable_type
-
-  expose :protected?, as: :protected
-  expose :masked?, as: :masked
+class GroupVariableEntity < Ci::BasicVariableEntity
 end

app/serializers/variable_entity.rb

 # frozen_string_literal: true
 
-class VariableEntity < Grape::Entity
-  expose :id
-  expose :key
-  expose :value
-  expose :variable_type
-
-  expose :protected?, as: :protected
-  expose :masked?, as: :masked
+class VariableEntity < Ci::BasicVariableEntity
   expose :environment_scope
 end