De-couple focal-point selection from crop-selection (image hotspot option)

[ryami333]

We have a use-case where the crop of an image is defined by the front-end, but we want to permit editors to define the focal-point of the image. Unfortunately, the hotspot feature is catch-all that enables both crop-selection and focal-point selection, which has been confusing our editors. We've been telling them just to ignore the cropping square, but it confuses them because image previews in Sanity itself use this crop selection.

This could be configured in a backward compatible manner by permitting boolean values and discrete options with a nested object, eg:

defineField(
    {
      name: "myImage",
      type: "image",
      title: "My Image",
      options: {
        // Configuration option #1: boolean
        hotspot: true,
        // Configuration option #2: discrete options
        hotspot: {
          crop: true,
          focalPoint: true,
        },
      },
    },
);