Alpaca/LBank-style subscribe frame keyed on an "action" field with a
params object.
%{"action" => "subscribe", "params" => %{"channels" => ["ticker"]}}Config keys:
:op_field— default"action":args_field— default"params":channels_key— key under which the channel list is placed within the params object (default"channels")
Not wired in CCXT.WS.Config today (no priority-tier exchange uses it).