Combine predefined activestorage attachment variants

[mdh]

Motivation / Background

Sometimes you will want to combine predefined-variants, like rotate and thumb:

@user.avatar.variant(:thumb, :rotate_90)

Until now, you could only use 1 variant. If you would like to combine multiple variants, you would have to explicitly create all possible combinations, which becomes quite cluttered quick, e.g.:

attachable.variant :thumb, ...
attachable.variant :thumb_rotate_90, ...
attachable.variant :thumb_rotate_180, ...

This PR adds support for combining multiple variants.

This Pull Request has been created because we use activestorage a lot and sometimes want to combine predefined-variants.

Detail

This Pull Request changes the way that ActiveStorage::Attachment looks up transformations. It is backwards compatible, but allows users to define more than 1 predefined variant.

Before:

has_one_attached :avatar do |attachable|
  attachable.variant :thumb,            resize_to_limit: [100, 100]
  attachable.variant :thumb_rotate_90,  resize_to_limit: [100, 100], rotate: 90
  attachable.variant :thumb_rotate_180, resize_to_limit: [100, 100], rotate: 180
end

@user.avatar.variant(:thumb_rotate_90)

After:

has_one_attached :avatar do |attachable|
  attachable.variant :thumb, resize_to_limit: [100, 100]
end

@user.avatar.variant(:thumb, rotate: 90)

Additional information

I chose to use some recursion in the lookup process, it works just fine, but might be hard to read for some. Could write this out in a more explicit way.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• There are no typos in commit messages and comments.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Feature branch is up-to-date with main (if not - rebase it).
• Pull request only contains one commit for bug fixes and small features. If it's a larger feature, multiple commits are permitted but must be descriptive.
• Tests are added if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• PR is not in a draft state.
• CI is passing.