Support images in AiMigrator

I've almost got the module migrating an image into Files. It was working earlier today, but I somehow broke it -- I think I've done something to slice1_image_files and the file_copy plugin is not getting a parameter it needs. See attached screenshot.

Nevertheless, it would be great to get a review of what's there thus far. Also, if someone else wants to pick it up and add commits, please do!

Also, I know PHPUnit tests are failing ... I didn't get a chance to update those since the last MR. If anyone wants to take that on, have at!

I updated the tests and left comments on a few things I noticed.

Noting here that I merged in @webbywe's branch into this issue's fork. Thank you for your contribution on stubs, webbywe! That looks like a more logical way to go, much less confusing than starting migration at the bottom (files) and working up through media, to content.

I think we still need to figure out how to get the migration to create the image urls from the stubs correctly, so I'm leaving this one open.

We also have some failing tests to fix.

Approved the code changes

Ran the following to test it.

1. ddev poser
2. ddev drush cim
3. ddev drush migrate:import slice1_content_with_stubs
4. Log in and confirm there are 2 new pieces of content.
5. ddev drush migrate:rollback --group=slice1 to roll it back.

Merging the changes and closing issue as fixed.