feat(fields): adds upload's relationship thumbnail

[rklos]

Description

I've done an implementation of the feature requested here: #3407

Before:

After:

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes

[rklos]

Maybe we should add this as an option in the config?

[revnelson]

Maybe we should add this as an option in the config?

Yeah, in the referenced post I was thinking that this should be the default behavior for upload relationships (unless someone can explain why some other option would be a more generally-desired display) with options to set any field on that relationship instead.

It'd also be nice if the File and Thumbnail components were exported to be used with documentation on how to use them, because I didn't even know where those were until this pull request (thanks, BTW!). I found them and imported them to use manually in the admin>components>Cell field, but after some logging I can see that only the related ID is available in the row data (not the full object) so the thumbnail isn't working.