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

Comece em src/parser.rs, por volta da linha 1322, onde AT TIME ZONE atualmente espera uma string entre aspas simples. Verifique as consultas de exemplo que usam uma coluna após a palavra-chave e confirme que tanto a forma literal quanto a forma com coluna são analisadas com sucesso.

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