API: expose created date for project hooks and user SSH keys

CHANGELOG

 v 4.0.0
+  - [API] expose created date for hooks and SSH keys
   - [API] list, create issue notes
   - [API] list, create snippet notes
   - [API] list, create wall notes

lib/api/entities.rb

@@ -14,7 +14,7 @@ module Gitlab
     end
     class Hook < Grape::Entity
-      expose :id, :url
+      expose :id, :url, :created_at
     end
     class Project < Grape::Entity
@@ -61,7 +61,7 @@ module Gitlab
     end
     class SSHKey < Grape::Entity
-      expose :id, :title, :key
+      expose :id, :title, :key, :created_at
     end
     class MergeRequest < Grape::Entity