Allow entity_generate to run process pipelines on values

Problem/Motivation

I am using Entity Generate to generate taxonomy terms on the fly, which seems like a common use case. My terms also have an image attached, which I also need to import.

The usual way of doing this is something like

process:
  _location_image:
    plugin: image_import
    source: imageUrl
    destination: constants/image_destination
    alt: alternateText

  field_location:
    plugin: entity_generate
    entity_type: taxonomy_term
    bundle_key: vid
    bundle: location
    value_key: name
    values:
      field_image: '@_location_image'

However this imports the image for every row, even if the term already exists.

Steps to reproduce

Proposed resolution

I propose extending values so if the value is an array it is treated as a process plugin; this allows additional processing on other fields, but that is only run when the entity is created. For example:

process:
  field_location:
    plugin: entity_generate
    entity_type: taxonomy_term
    bundle_key: vid
    bundle: location
    value_key: name
    values:
      description/value:
        plugin: snippet
        module: custom_migrate
        path: clean_html
        source: locationDescription
      description/format:
        plugin: default_value
        default_value: basic_html
      path/alias:
        plugin: str_replace
        regex: true
        source: locationUrl
        search: '/^https?:\/\/[^\/]+/'
        replace: ''
      path/pathauto:
        plugin: default_value
        default_value: false
      field_image:
        plugin: image_import
        source: imageUrl
        destination: constants/image_destination
        alt: alternateText

Remaining tasks

User interface changes

API changes

Data model changes