Keywords AT TIME ZONE can be followed by a column

We have queries which reference a column after `AT TIME ZONE` rather than just supplying a string. This results in a valid query which executes as expected (in PostgreSQL). The parser currently raises an error `Expected Token::SingleQuotedString after AT TIME ZONE, found: the_column`.

For example, instead of `SELECT datetime_column AT TIME ZONE 'America/Denver' FROM table` it is also valid to have `SELECT datetime_column AT TIME ZONE zone_column FROM table`

I believe further match conditions should be added here to patch: https://github.com/sqlparser-rs/sqlparser-rs/blob/main/src/parser.rs#L1322

从 src/parser.rs 第 1322 行附近开始，这里 AT TIME ZONE 目前要求使用单引号括起来的字符串。检查在关键字后使用列的示例查询，并验证字面量形式和列形式都能成功解析。

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