File upload: use preview options

[distantnative]

Description

Summary of changes

• Upload item preview supports back and cover props
• New $panel.upload.preview object that can hold defaults for the upload item preview component
• Files field and files section pass their image options to $panel.upload.preview

Reasoning

• Likely, the options that one uses to make the preview in the field/section better also make the preview in the upload dialog better, as they consider what kind of files are likely uploaded (e.g. black SVG might want to use a colored background)

Changelog

Enhancements:

• File uploads: preview uses image options from field/section
  [v4] Upload dialog: use field settings for preview #5729

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion

[bastianallgeier]

After setting up the lab examples, it felt better to keep the upload item and upload items components independent from the upload.js module. @distantnative could you please have a second look if this is alright for you?

I've also added new blueprint setups for the fields to the sandbox repo.