Hyperliquid-style subscribe frame.
%{"method" => "subscribe", "subscription" => %{"type" => "allMids"}}The channel list must contain exactly one entry; its value becomes the
type field of the nested subscription object. Config keys:
:op_field (default "method"), :args_field (default "subscription").
Hyperliquid is in the :custom DEX bucket and is not wired in
CCXT.WS.Config today — this module is ported for completeness but not
exercised end-to-end until the DEX custom track unlocks upstream.