Expose "reason for rejection in invoice API end point.

[Speedway1]

Background context When the invoice status changes to "rejected" there is a corresponding reason for the rejection entered into the "reason for rejection" field on Reapit. However, this field hasn't been exposed in the API meaning that invoice rejections cannot be meaningfully processed.

A large mutual customer has requested some efficiency automations from us to automatically contact contractors with a notification of the rejection and the reason for it. This would represent a very significant saving for the customer, however we need this value exposed before we can complete the process.

Specification Expose "reason for rejection" in the invoice details API end point.