Afform entities: Use mixin for 5.50+, add shim for < 5.50

See https://github.com/civicrm/civicrm-core/pull/23303

mixin/afform-entity-php@1/mixin.php

+<?php
+
+/**
+ * Empty shim, can be removed when this extension drops support for core < 5.50
+ *
+ * @mixinName afform-entity-php
+ * @mixinVersion 1.0.0
+ *
+ * @param CRM_Extension_MixInfo $mixInfo
+ * @param \CRM_Extension_BootCache $bootCache
+ */
+return function ($mixInfo, $bootCache) {
+
+  // This shim doesn't actually need to do anything because prior to CiviCRM 5.50,
+  // afformEntiteis were loaded automatically without any event.
+  // In CiviCRM 5.50+ this shim won't be used; the mixin from core will load the entities.
+
+};