Keywords AT TIME ZONE can be followed by a column

[jdclarke5]

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