update dunning_campaign.finished definition

[lovrocolic]

Multi-currency support: overdue_balances on dunning_campaign.finished webhook

What

Adds a new additive field overdue_balances to the dunning_campaign.finished
webhook payload to support multi-currency dunning. The field is an array breaking
down the overdue balance per currency.

Each item contains:

• amount_cents (integer)
• currency ($ref to the shared ISO 4217 Currency schema)

Why

With multi-currency support, a customer's overdue balance can span more than one
currency. The existing scalar overdue_balance_cents / overdue_balance_currency
fields can only express a single currency, so we add a per-currency breakdown
alongside them.

Backward compatibility

Fully additive. No fields renamed or removed. The existing
overdue_balance_cents and overdue_balance_currency fields are unchanged.