fix: Use isSortable flag to determine if colBy and rowBy sources are sortable

[vbabich]

Use isSortable flag to determine if colBy or rowBy source is sortable.

Tested by running this query and clicking on rowBy/columnBy headers to sort -

from deephaven_enterprise.pivot.pivot_table import make_pivot
from deephaven import agg
t = db.historical_table("LearnDeephaven","StockQuotes") \
    .where("Date=`2017-08-25`")
# AskExchange and BidExchange are StringSet columns, not sortable
pivot = make_pivot(
    t,
    [agg.avg(["Bid"])],
    ["USym", "AskExchange"],
    ["Exchange", "BidExchange"],
)

Split out the hooks file into separate files, based on a review comment from another PR.