Add "extra" field to display all footnotes

I created a patch following the issue description. It has the https://www.drupal.org/project/footnotes/issues/3098138 custom service in order to make the "extra" field work. Any comments or suggestions are welcome.

Hello,

I have tried this patch but I have the impression that the content of this extra field is empty. I can't display the footnotes group on the node.
In the footnotes_preprocess_node() function, the field_footnotes_group is not added because $footnotes_group->buildFooter()['#footnotes'] is empty.

I have tested the patch #29 in issue 3098138 (All footnotes available as block for Layout Builder / Paragraph / ...) in association with this current patch (only a part because patch could not be applied) : the footnotes group block gives some data, but the extra field given by this patch is empty.

Here is the part I used from the current patch, the other parts are already given by the patch in issue 3098138

diff --git a/footnotes.module b/footnotes.module
index ad38ad5..9d50845 100644
--- a/footnotes.module
+++ b/footnotes.module
@@ -89,3 +89,34 @@ function footnotes_theme() {
 function footnotes_is_footnotes_later($format, $caller_filter) {
   return $format['filter_footnotes']['weight'] > $caller_filter['weight'];
 }
+
+/**
+ * Implements hook_entity_extra_field_info().
+ */
+function footnotes_entity_extra_field_info() {
+  $extra = [];
+  $entityTypeManager = \Drupal::service('entity_type.manager');
+
+  $contentTypes = $entityTypeManager->getStorage('node_type')->loadMultiple();
+  foreach ($contentTypes as $contentType) {
+    $extra['node'][$contentType->id()]['display']['field_footnotes_group'] = [
+      'label' => t('Footnotes group'),
+      'description' => t('Footnotes group field.'),
+      'weight' => 102,
+      'visible' => TRUE,
+    ];
+  }
+
+  return $extra;
+}
+
+/**
+ * Implements hook_preprocess().
+ */
+function footnotes_preprocess_node(&$variables) {
+  $footnotes_group = \Drupal::service('footnotes.group');
+
+  if (!empty($footnotes_group->buildFooter()['#footnotes'])) {
+    $variables['content']['field_footnotes_group'] = $footnotes_group->buildFooter();
+  }
+}

Re-rolled patch against 3.1.0

Removed Patch created. No change. Uploaded same patch by mistake.

Going to merge this into 4x and created a follow-up here #3413181: Follow-up to all footnotes available as a block to apply to modal plugin to apply it to the new modal version before the old one gets retired. Patches still needed while on 3x until 4x (and its upgrade path are more stable).