If a variant has a large set of options associated with it, the generated
filename will be too long, causing Errno::ENAMETOOLONG to be raised. This
change replaces those potentially long filenames with a much more compact
SHA256 hash. Fixes#30662.
This fixes following warnings:
```
test/models/variant_test.rb:11: warning: ambiguous first argument; put parentheses or a space even after `/' operator
lib/active_storage/attached/macros.rb:63: warning: instance variable @active_storage_attached_highlights not initialized
lib/active_storage/attached/macros.rb:25: warning: instance variable @active_storage_attached_avatar not initialized
```