Add support for custom_field module

Created an MR that updates custom_field data in the Default Content export for referenced entities.

Original output without this MR:

field_my_custom_field:
    -
      label: 'Exported label'
      image: 21 # Some random file ID
      image__width: 1024
      image__height: 1024
      image__alt: Sunset
      image__title: ''
      file: 22 # Some random file ID
      term: 4 # Some random taxonomy term ID

After applying this MR. this becomes:

  field_my_custom_field:
    -
      label: 'Exported label'
      image: aa6340f0-0006-4f31-9f2d-5595f5cce028
      image__width: 1024
      image__height: 1024
      image__alt: Sunset
      image__title: ''
      file: c36ea199-cd17-4fe5-bec7-beef1ffb51b4
      term: 78ae1ee7-f2d4-4fc0-871e-1af92730df3b

The referenced UUIDs also get added as a dependency to the "_meta" definition.

On the import side of things, there is a patch for custom_field that makes this work: 📌 Support Drupal core DefaultContent API Active .

Attached is a patch file for the current state of the MR, useful for Composer based patching workflows.

@svendecabooter,

Building on my comment in https://www.drupal.org/project/custom_field/issues/3528586#comment-16166705 📌 Support Drupal core DefaultContent API Active , it might just make best sense to create a reference type base class in custom_field that those 3 types extend so that this patch here can just depend on a single class. Forward thinking here in case we were to add new similar data types in custom_field so we don't have to go back and request a change again here.

This patch would put a dependency on custom_field module. Needs to just check for the class as string vs. requiring the interface.