Move the decision "DebitNote/Invoice -> accepted amount" to the MarketStrategy

[johny-b]

Something like:

class MarketStrategy:
    async def accepted_amount(document: Union[DebitNote, Invoice]) -> float:
        # current "default" implementation
        return document.amount

Additional things that could be done within this issue:

• Why do we have Invoice.amount and DebitNote.total_amount_due? Is there any meaningful difference?
• Why do we accept invoices in two different places in yapapi.engine? Maybe this could be refactored?