Support postgres types transparently in queries

[Kikkon]

Hi 👋
I'm following up on this: apache/datafusion#6817 and found out that some changes need to be made to sqlparser-rs, such as adding some new alias types in DataType, this change could also help sqlparser-rs to be more compatible with postgres sql, any other better suggestions? : )

[Kikkon]

Another problem: the UNSIGNED keyword doesn't exist in postgres, and BIGINT(20) isn't supported, but BIGINT in sqlparser supports both, so it seems that if we use INT8 as an alias for BIGINT, it would have to support both.