Deprecate Paperclip Adapter

[kennyadsl]

Summary

Closes #5033

Add solidus:paperclip_adapter:install generator, which allows incorporating the paperclip adapter into the host application, in order to remove support for it from the core.

This generator does two things:

1. Copy the Image and Taxon Paperclip adapters into the host application;
2. Set the right configurations in the spree.rb (path configurable) initializer in order to use the newly created files.

This PR also deprecates the Paperclip adapters, so that we can remove it in the next major version.

TODOs

• [ ] install gem on the host app's Gemfile because we will remove it from dependencies.

Checklist

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[kennyadsl]

I think we need to change CI for the paperclip adapter so the generator is run, right? 🤔

At the moment, the paperclip CI jobs should use the old adapter, which I don't think it's working because tests are failing and I don't understand why. Your proposed approach seems interesting, this way we'll stop using the deprecated adapter and just rely on the generated files. I'll give it a try.