Add support for CDN for Spina::Parts::Image and Attachment

[Bramjetten]

ActiveStorage requires setting a custom host if you want to use a CDN. We already have a list of resolve calls in routes.rb to handle urls for Spina::Parts::Image and Spina::Parts::ImageVariant.

We can add options.merge(host: Spina.config.cdn_host) or something similar to easily add support for CDNs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.