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

Start in src/parser.rs around line 1322, where AT TIME ZONE currently expects a single-quoted string. Check the example queries using a column after the keyword and verify that both literal and column forms parse successfully.

Written by the indexing model from the issue text.