Create an ActiveStorage adapter

[kennyadsl]

At the moment Solidus only supports Paperclip as attachments system.

We started some work to make it pluggable in solidusio#3237 so that it would be easier to add ActiveStorage support when the blocking public blob URLs issue is fixed.

Now that that work has been done (rails/rails@94584c2), and it will be released with Rails 6.1, we should resume the work done in solidusio#2974 and try to add an ActiveStorage adapter to Solidus.

This issue includes:

• adding the ActiveStorage adapter
• making it available to Rails 6.1+ users
• testing on a real store
• adding a Guides page on how to use the new adapter
• considering start shipping the new adapter as default in new Solidus installations
• providing guidance (wiki page?) to migrate from Paperclip to ActiveStorage, if possible