This adds a data transform plugin system to Paragraphs Sets by wrapping that module's data_alter hooks.

The annotated plugins operate kinda sorta like migration process plugins. Currently include plugins:

• simple: an identity plugin
• create_entity: construct nested entities for an entity reference field (deprecated in favor of nested_entities)
• nested_entities: recursive nested entity construction; handles nested paragraphs

For example, in a paragraphs set, we want a paragraph that references custom entities to prepopulate (i.e., create) the entities:

...
-
    type: fast_facts
    data:
      field_heading: 'Why Choose This Degree?'
      field_nested_paragraphs:
        plugin: nested_entities
        data:
          -
            bundle: fast_fact
            field_heading: 'Test fact 1'
            field_body: 'Lorem ipsum'
          -
            bundle: fast_fact
            field_heading: 'Test fact 2'
            field_body: 'Lorem ipsum'
...

The field_collection_items field gets populated using the nested_entities plugin, while field_heading gets populated as before using the paragraphs_sets builtin field value setter.