Add support for extending a window definition

[johnedquinn]

Relevant Issue/Bug

• Adds RANK, DENSE_RANK, LAG, LEAD, & ROW_NUMBER Window Functions & Operator #1746

Requested Solution/Feature

We should eventually support being able to extend a window definition. Something like:

SELECT
   RANK OVER (w1 PARTITION BY xx ORDER BY xx) -- extending w1 here.
FROM t
WINDOW
  w1 AS (PARTITION BY t.a ORDER BY t.b)

I haven't gone through the spec to understand how much one can do with a particular window (can you both partition and order a window?), but this is something to look into.