PIM-7982: Add reference entity attribute resources

Author: LaurentPetard

content/php-client/resources/reference-entity/attributes.md

@@ -0,0 +1,59 @@
+### Reference entity attribute
+
+::: warning
+This resource is only available in the [Entreprise Edition](https://www.akeneo.com/enterprise-edition/).
+:::
+
+#### Get an attribute of a given reference entity
+
+```php
+$client = new \Akeneo\PimEnterprise\ApiClient\AkeneoPimEnterpriseClientBuilder('http://akeneo.com/')->buildAuthenticatedByPassword('client_id', 'secret', 'admin', 'admin');
+
+/*
+ * Returns an array like this:
+ * [
+ *     'code' => 'description',
+ *     'labels' => [
+ *         'en_US' => 'Description',
+ *         'fr_FR' => 'Description',
+ *     ],
+ *     'type' => 'text',
+ *     'localizable' => true,
+ *     'scopable' => true,
+ *     'is_required_for_completeness' => true,
+ *     'max_characters' => null,
+ *     'is_textarea' => true,
+ *     'is_rich_text_editor' => true,
+ *     'validation_rule' => 'none',
+ *     'validation_regexp' => null,
+ * ]
+ */
+$referenceEntityAttribute = $client->getReferenceEntityAttributeApi()->get('brand', 'description');
+̀ ``
+
+#### Get the list of attributes of a given reference entity
+
+```php
+$client = new \Akeneo\PimEnterprise\ApiClient\AkeneoPimEnterpriseClientBuilder('http://akeneo.com/')->buildAuthenticatedByPassword('client_id', 'secret', 'admin', 'admin');
+
+$referenceEntityRecordsCursor = $client->getReferenceEntityAttributeApi()->all('brand');
+```
+
+#### Upsert an attribute of a given reference entity
+
+If the attribute does not exist yet, this method creates it, otherwise it updates it.
+
+```php
+$client = new \Akeneo\PimEnterprise\ApiClient\AkeneoPimEnterpriseClientBuilder('http://akeneo.com/')->buildAuthenticatedByPassword('client_id', 'secret', 'admin', 'admin');
+
+$client->getReferenceEntityAttributeApi()->upsert('brand', 'description', [
+    'code' => 'description',
+    'labels' => [
+        'en_US' => 'Description'
+    ],
+    'type' => 'text',
+    'localizable' => true,
+    'scopable' => true,
+    'is_required_for_completeness' => false
+]);
+```