refactor: prepare to decouple the "model migration" package and "models" package (#38533)

Migrations should never use model structs directly, because the model
structs can be different in different releases. e.g. if one migration uses
"User" model, it works in the early releases, then one day, when the
User model changes, the migration breaks because it will use the
new (incorrect) User model, it should only use the old User model.

The same to "modules/structs".

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

modelmigration/v1_20/v244.go

@@ -0,0 +1,20 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_20
+
+import "gitea.dev/modelmigration/base"
+
+func AddNeedApprovalToActionRun(x base.EngineMigration) error {
+	/*
+		New index: TriggerUserID
+		New fields: NeedApproval, ApprovedBy
+	*/
+	type ActionRun struct {
+		TriggerUserID int64 `xorm:"index"`
+		NeedApproval  bool  // may need approval if it's a fork pull request
+		ApprovedBy    int64 `xorm:"index"` // who approved
+	}
+
+	return x.Sync(new(ActionRun))
+}