Add memory/disk usage for WindowAgg nodes in EXPLAIN

1eff8279d4 added memory/disk usage for materialize nodes in EXPLAIN
ANALYZE.
In the commit message:
> There are a few other executor node types that use tuplestores, so we
> could also consider adding these details to the EXPLAIN ANALYZE for
> those nodes too.

So I wanted to Add memory/disk usage for WindowAgg. Patch attached.

Since WindowAgg node could create multiple tuplestore for each Window
partition, we need to track each tuplestore storage usage so that the
maximum storage usage is determined. For this purpose I added new
fields to the WindowAggState.
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Tatsuo Ishii
SRA OSS LLC
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