Allow defining images sizes where they are used (in the relationship field)

[tylkomat]

I would be great if imagesizes could be defined where they are used. It's fine to have a global setting for thumbnail sizes, but mostly I don't need all my images in all defined sizes.

If you have for example a hero block which would be 1920x400 and a block for the members of your team which are 300x300. These images are not interchangeable. I won't put some partial view (just the eyes) of a team member in the hero block, and the other way around, a hero image which is wide is usually not design to go into a square block.

It could probably go under admin.upload when then relationship field points to an upload enabled collection.

{
  slug: 'team'
  fields:  [
    {
      name: 'image',
      type: 'upload',
      relationTo: 'photos',
      admin: {
        upload: {
          imagesizes: [{
            width: 300,
            height: 300,
            position: 'center',
            fit: 'inside',
            formatOptions: {
              format: 'webp',
            },
          }]
        }
      },
    }
  ]
}

When uploading a new image via this relationship field, the "global" imagesizes (defined in the relationTo collection) are merged with what was defined in the relationship field and the corresponding sizes are generated.
When picking an existing image it is resized based on the sizes defined in the relationship field, if the image does not already exist in those sizes.