Expose exchange rate in Customer Creditnote

[Eirik Morken - Proplan AS]

We would like the exchange rate field to be exposed in the v2/customerCreditNote endpoint

This is already exposed in both v2/customerinvoice and v2/customerDebitNote endpoints.

This case therefore seems more like a forgotten field in the process of implementing v2/customerCreditNote as its twin endpoints has this field. I would at least expect the same fields available in all these three endpoints.