Add method to API::Variables for filtering params

This allows EE to customize the parameters used in two places, without
having to modify the source code directly.

lib/api/variables.rb

@@ -7,6 +7,14 @@ module API
     before { authenticate! }
     before { authorize! :admin_build, user_project }
 
+    helpers do
+      def filter_variable_parameters(params)
+        # This method exists so that EE can more easily filter out certain
+        # parameters, without having to modify the source code directly.
+        params
+      end
+    end
+
     params do
       requires :id, type: String, desc: 'The ID of a project'
     end
@@ -50,6 +58,7 @@ module API
       end
       post ':id/variables' do
         variable_params = declared_params(include_missing: false)
+        variable_params = filter_variable_parameters(variable_params)
 
         variable = user_project.variables.create(variable_params)
 
@@ -75,6 +84,7 @@ module API
         break not_found!('Variable') unless variable
 
         variable_params = declared_params(include_missing: false).except(:key)
+        variable_params = filter_variable_parameters(variable_params)
 
         if variable.update(variable_params)
           present variable, with: Entities::Variable