增加apply方法，用于拼接自定义sql字符串

需求案例场景：子查询、自定义sql片段
伪代码：
```go
	var expectSql = "SELECT * FROM `Users` WHERE username = 'afumu' OR age = 20"
	sessionDb := checkSelectSql(t, expectSql)
	query, u := gplus.NewQuery[User]()
	query.Eq(&u.Username, "afumu").Or().Eq(&u.Age, 20).Apply("AND id IN (SELECT user_id FROM app_user)")
	gplus.SelectList[User](query, gplus.Db(sessionDb))
```