Corrected "admin_configurations" schema on the trading account response

Broker API

Corrects the admin_configurations schema on the trading account response so it reflects what the endpoint actually returns:

max_margin_multiplier is now documented as a string ("1", "2", or "4") instead of a {from, to} object.
max_options_trading_level is now documented as an integer with values 0 (disabled), 1 (Covered Call/Cash-Secured Put), 2 (Long Call/Put), and 3 (Spreads/Straddles) instead of a {from, to} object.
user_configurations is now nullable, since it is null until user-level overrides are set.
The {from, to} change-object shape now lives in a separate AdminConfigurationsEvent schema used by the account status events stream.