Add a basic placeholder field

Adds a basic `commented_by` field to the merge request
approvals data. This will be populated in a later
iteration.

lib/api/entities/merge_request_approvals.rb

@@ -18,6 +18,10 @@ module API
       expose :approved_by, using: ::API::Entities::Approvals do |merge_request|
         merge_request.approvals
       end
+
+      expose :commented_by, using: ::API::Entities::Approvals do |merge_request|
+        []
+      end
     end
   end
 end

spec/lib/api/entities/merge_request_approvals_spec.rb

@@ -21,7 +21,8 @@ RSpec.describe API::Entities::MergeRequestApprovals do
       approved: true,
       approved_by: [{
         user: API::Entities::UserBasic.new(user).as_json
-      }]
+      }],
+      commented_by: []
     })
   end
 
@@ -30,7 +31,8 @@ RSpec.describe API::Entities::MergeRequestApprovals do
       user_has_approved: false,
       user_can_approve: true,
       approved: false,
-      approved_by: []
+      approved_by: [],
+      commented_by: []
     })
   end
 end