Get the RFQ operator fee rates, so a rate can be checked before raising a request.
Rates are fractions (0.0025 = 25 bps); a rate that has never been configured
reads as 0. The schedule is platform-wide; there are no per-account rates.

Spot keys follow rfq_spot_<class>_<maker|taker>_fee. The requester pays the
operator fee, at the taker rate.

Pass product=spot to get only the spot schedule; asset_class and side narrow
it further, down to a single key when all three are given. An unknown value is a
400 so a typo never reads as "no fee".