Add PRODUCT() aggregate and window function

[lukaseder]

Most SQL implementations don't have a PRODUCT() aggregate and window function, but it can be emulated easily as such:

-- Aggregate function
exp(sum(log(arg)))

-- Window function
exp(sum(log(arg)) OVER window_specification)

This only works for strictly positive numbers. If zero and negative numbers should be supported, this approach could be used:

WITH v(i) AS (VALUES (-2), (-3), (-4), (-5))
SELECT 
  CASE 
    WHEN SUM (CASE WHEN i < 0 THEN -1 END) % 2 < 0 
    THEN -1 
    ELSE 1 
  END * EXP(SUM(LN(ABS(i)))) multiplication2
FROM v;

See: https://blog.jooq.org/2018/09/21/how-to-write-a-multiplication-aggregate-function-in-sql

[lukaseder]

First implementation done. Open points:

• The implementation is not zero-safe (logarithm of zero undefined)
• The implementation might not get FILTER() clause right
• Parser support

[lukaseder]

Any open issues will be fixed as bugs