Add admin.transform property for fields

[r1tsuu]

Proposal: add admin.transform property for fields. This should run in the same place with condition, validate and defaultValue on the server, on every /form-state request. This way, we can transform field values in a real time, without needing for autosave, hooks and custom Field components.

Example of the usage for a slug field:

const Pages: CollectionConfig = {
  slug: 'pages',
  fields: [
    {
      name: 'title',
      type: 'text',
    },
    {
      name: 'slug',
      type: 'text',
      admin: {
        transform: ({ data, siblingData, value, req }) => {
          if (siblingData.editSlug) return value
          return slugify(siblingData.title)
        },
      },
    },
    {
      name: 'editSlug',
      type: 'checkbox',
    },
  ],
}