新增LoadMigrationFiles事件，支持扩展提供的数据库迁移文件

extend/think/migration/command/Migrate.php

@@ -104,6 +104,14 @@ abstract class Migrate extends Command
         if (null === $this->migrations) {
             $phpFiles = glob($this->getPath() . DIRECTORY_SEPARATOR . '*.php', defined('GLOB_BRACE') ? GLOB_BRACE : 0);
 
+            $event_files = event_handle_result('LoadMigrationFiles','file_list');
+
+            foreach ($event_files as $file_list) {
+                foreach ($file_list as $file_path) {
+                    $phpFiles[] = $file_path;
+                }
+            }
+
             // filter the files to only get the ones that match our naming scheme
             $fileNames = [];
             /** @var Migrator[] $versions */