Add index on group_content_field_data (type, entity_id)

I would definitely be up for committing any patch that adds said index to the database. Currently a bit busy with the query access handler work, so a head-start would be appreciated :)

I have added the storage_schema, which provides the required index. I added an update hook, which add storage schema class to group content, but I couldn't find the way how to run update of the entity in order to apply the schema changes.
I do know how to update the field, when we add an index (addSharedTableFieldIndex) for specific field ()

$entity_definition_update_manager = \Drupal::entityDefinitionUpdateManager();
$entity_definition_update_manager->updateFieldStorageDefinition($entity_definition_update_manager->getFieldStorageDefinition('field_name', 'fast_node'));

In our case it is a composite index.
If somebody knows, how to do it, please let me know I will update the patch

@Berdir, thank you for all your help. Debug helped me to understand, that I just missed use statement for GroupContentStorageSchema class and it was never used, but you are right no manual adding of the index is required.

+++ b/src/GroupContentStorageSchema.php
@@ -0,0 +1,28 @@
+    $schema = parent::getEntitySchema($entity_type, TRUE);

Why TRUE instead of passing on $reset?

I've changed this to $reset for now, rerolled the patch and moved a few files/comments around.

Ideally, this would have a test proving that the update works. But if anyone could point me to a core test showing that the used functionality indeed updates just the indexes, then I'm fine committing this without an update hook test.

@kristiaanvandeneynde. Yes, I have forgotten to remove TRUE, it was done for a test purpose.
I have checked other cases in the core and I didn't any tests for indexes
Some examples:
https://www.drupal.org/project/drupal/issues/2183231
https://www.drupal.org/project/drupal/issues/3007669

Fixed, thanks!