Support postgresql on commit for ctas

Add support for postgresql `[ ON COMMIT { PRESERVE ROWS | DELETE ROWS | DROP } ]`. Currently panics:

```
use sqlparser::dialect::PostgreSqlDialect;
use sqlparser::parser::Parser;

fn main() {
let sql = "create local temporary table test on commit preserve rows as select 1 as a;";

let dialect = PostgreSqlDialect {};

let ast = Parser::parse_sql(&dialect, sql).unwrap();

for stmt in ast.iter() {
println!("{}", stmt.to_string())
}
}
```

`ParserError("Expected end of statement, found: as at Line: 1, Column 59")`

从复现中展示的 PostgreSqlDialect 和 Parser::parse_sql 入口点开始，然后运行提供的 SQL 示例。完成的标准是：语句能够成功解析和格式化，同时支持 ON COMMIT 后的 PRESERVE ROWS、DELETE ROWS 和 DROP。

由索引模型根据 Issue 内容生成。