Get fee tiers. filter=perps|spot returns the regular user tiers (both market types share them). filter=bo returns BO volume tiers and LIT staking discount tiers; filter=all returns everything. An account's BO multiplier is the BO tier fee_rate times the staking fee_rate, applied to each BO market's maker/taker fee (see /api/v1/events) and then scaled by 1 - trade_price / settlement_cap.