Support FILTER emulation with Oracle's KEEP syntax

[lukaseder]

This hypothetical syntax should work with jOOQ:

select sum(x) 
  filter (where z != 1)
  keep (dense_rank first order by y) 
from t

It currently doesn't parse:

Keyword 'SELECT' expected: [3:9] select sum(x) 
  filter (where z != 1)
  keep ([*]dense_rank first order by y) 
from t

So, let's get this to work as well. Once it works it should be emulated as follows:

select sum(case when z <> 1 then x end)
  keep (dense_rank first order by case when z <> 1 then y end nulls last)
from t

It currently produces this emulation, though, which is "wrong" (or let's say, less useful):

select sum(case when z <> 1 then x end)
  keep (dense_rank first order by y)
from t

The FILTER should apply also to the KEEP clause.

This is a backwards incompatible change. I'm not 100% convinced we should implement it, yet. Specifically, because in jOOQ, the keepDenseRankFirst() clause is appended to the AggregateFunction first, before the filterWhere() clause, which hints at the current implementation being correct.

But at least it's documented now as a tempting change, which can be discussed.