Attempt to find nested/referenced text field values for content suggestions

Problem/Motivation

The AI Content Suggestions module currently looks for text fields in the node form. There are instances where it would be valuable to extract text fields from referenced items in the form. For example, an associated Media item with an extracted text field of the uploaded document could be used to suggest content. Another example would be the description fields on referenced taxonomy terms.

There's already similar handling for Paragraph entities:

                // If we already have a paragraph of the correct type, just
                // reuse it.
                $paragraph = ($paragraph && $paragraph->bundle() == $bundle) ? $paragraph : $this->entityTypeManager->getStorage('paragraph')
                  ->create([
                    'type' => $bundle,
                  ]);

                foreach ($this->getAllTextFields($paragraph, $form[$name]['widget'][$key]['subform']) as $machine => $label) {
                  $identifier = implode(':', [$name, $key, 'subform', $machine]);
                  $options[$identifier] = $field->getLabel() . ' (' . $key . ') > ' . $label;
                }

Steps to reproduce

Proposed resolution

Attempt to load referenced entities and add any data from the fields on that entity that are text fields.

Remaining tasks

User interface changes

API changes

Data model changes