Allow paths in plugin.json publish.image field

[NGnius]

Currently the image field only accepts links to an image, but they are all getting uploaded to the store CDN anyway.
It would make sense to also accept a file path to an image contained within the plugin's git repo since the link is only used by the CI anyway.

For a bit more context (discord link to SD Homebrew guild): https://discord.com/channels/960281551428522045/960284311444131840/995760749764935790

[TrainDoctor]

@gbdlin is now working on store features that would provide this functionality.