New "fee_inclusive" field on funding wallet withdrawals

Broker API

Adds an optional fee_inclusive boolean to the funding wallet withdrawal request schema.

When fee_inclusive is true, fees are deducted from usd_amount instead of being added on top, so the total debited from the account equals usd_amount exactly and the beneficiary receives usd_amount minus fees.
This allows draining an account to zero by passing its full available cash as usd_amount.
The request is rejected if usd_amount is less than or equal to the fees.
Defaults to false, where fees are added on top of usd_amount, so existing behavior is unchanged.