api-platform: enable the client pagination config

Mainly so it renders the relevant pagination parameters into the
openapi and api docs. We handle the parameters manually in all cases.
Instead of "itemsPerPage" we default to "perPage", for historic reasons.

src/DependencyInjection/DbpRelayCoreExtension.php

@@ -60,6 +60,15 @@ class DbpRelayCoreExtension extends ConfigurableExtension implements PrependExte
 
         $container->prependExtensionConfig('api_platform', [
             'title' => $config['docs_title'],
+            'collection' => [
+              'pagination' => [
+                  // This enables it for the doctrine integration, which we don't actually use.
+                  // But it also adds it to the open-api docs which need because we implement it manually
+                  // in the controllers and providers, so enable it anyway.
+                  'client_items_per_page' => true,
+                  'items_per_page_parameter_name' => 'perPage',
+              ],
+            ],
             'description' => $config['docs_description'],
             'defaults' => [
                 'cache_headers' => [