Include group and project created_at attributes in file exports for offline transfer

What does this MR do and why?

Include group and project created_at attributes in file exports for offline transfer

References

closes: #592138 #592137

Screenshots or screen recordings

N/A

How to set up and validate locally

Spec checks

bundle exec rspec \
  spec/lib/import/offline/groups/transformers/group_attributes_transformer_spec.rb \
  spec/lib/import/offline/groups/pipelines/group_pipeline_spec.rb \
  spec/lib/import/offline/projects/pipelines/project_pipeline_spec.rb \
  spec/lib/gitlab/import_export/group/tree_saver_spec.rb \
  spec/lib/gitlab/import_export/group/tree_restorer_spec.rb \
  spec/lib/gitlab/import_export/project/tree_saver_spec.rb \
  spec/lib/gitlab/import_export/attributes_permitter_spec.rb \
  ee/spec/lib/ee/gitlab/import_export/project/relation_tree_restorer_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #592138