Add support for GROUP BY DISTINCT

[lukaseder]

A future version of PostgreSQL might implement the standard SQL GROUP BY DISTINCT clause, which helps remove redundant GROUPING SETS:
https://www.postgresql.org/message-id/E1lMwMS-0007AP-Qu%40gemulon.postgresql.org

Let's add support for it in jOOQ, and investigate what other dialects support it natively (given that it's standard SQL), or how it can be emulated, see #11669

[lukaseder]

There's no need to add support for GROUP BY ALL in the API, as that is equivalent to GROUP BY (we also don't support SELECT ALL or UNION DISTINCT). However, the parser can also parse GROUP BY ALL