Add application setting to permit MinIO for offline transfers

What does this MR do and why?

Add application setting to permit MinIO for offline transfers

Introduces a new application setting allow_minio_for_offline_transfer to control whether MinIO can be used as an object storage location for offline transfers. This replaces the previous Rails.env.development? check with a proper configurable setting.

Changes:

• Add allow_minio_for_offline_transfer boolean to the importers jsonb column in ApplicationSetting model (defaults to false)
• Add JSON schema validation for the new setting
• Update Import::Offline::Configuration to use the new setting instead of Rails.env.development?
• Update specs to use stub_application_setting

I have not yet made this setting configurable via the API or documented it. I will address that in a follow-up MR.

References

• #579705
• https://docs.gitlab.com/development/application_settings/

How to set up and validate locally

ApplicationSetting.current.update!(allow_minio_for_offline_transfer: false)
Import::Offline::Configuration.new.send(:supported_providers)
=> ["aws"]

ApplicationSetting.current.update!(allow_minio_for_offline_transfer: true)
Import::Offline::Configuration.new.send(:supported_providers)
# => ["aws", "minio"]

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