Improve postgres CREATE FUNCTION support

Hey,

I'm trying to parse `CREATE FUNCTION` statements (PostgreSQL) with the following attributes:

```sql
CREATE OR REPLACE FUNCTION public.accounting_ids()
RETURNS SETOF uuid
LANGUAGE plpgsql
STABLE SECURITY DEFINER
SET search_path TO ''
AS $function$
```

A few features fail to parse:
1. `SETOF [type]` is not supported (Expected: end of statement, found: uuid at Line: 5, Column: 16)
2. `SECURITY DEFINER` is not supported (Expected: end of statement, found: SECURITY at Line: 7, Column: 9)
3. `SET search_path TO ''` is not supported (Expected: end of statement, found: SET at Line: 7, Column: 2)

Using sqlparser 0.50.0

使用 sqlparser 0.50.0 重现 PostgreSQL 的 CREATE FUNCTION 示例，并定位 CREATE FUNCTION 的 parser 入口点。跟踪返回类型和函数属性的处理方式，然后为 SETOF uuid、SECURITY DEFINER 和 SET search_path TO '' 添加覆盖。完成标准是这三个构造都能成功解析。

由索引模型根据 Issue 内容生成。