Debit response should return Balanced fee

[charlietran]

When debiting a card or bank account, the merchant fee charged by Balanced is not included in the API response. There is a fee field, but that is always blank and appears to be deprecated.

I suggest that fee be reinstated in some form, ideally as an integer for the Balanced fee in cents, for the following reasons:

1. It would allow an app to store the charge fees more atomically. This is very helpful when the app's checkout process allows other providers to be used (Paypal, Amazon, etc) and the merchant fees can be stored at a per-transaction level.
2. In our case, our Balanced pricing has changed recently. When doing our monthly invoicing, this has put us into the unideal situation for calculating merchant fees based on transaction created_at times, because Balanced is the only provider we use that doesn't pass back fees at merchant time.
3. Rather than retrieving/reconciling fees from another API endpoint, it would be great to store the Balanced fees at the time of checkout rather than having to set up a scheduled worker for this purpose.

[mahmoudimus]

Relevant 2 issues: #233 #231 #130

[mahmoudimus]

also #185

[charlietran]

@mahmoudimus thoughts in response to the points above?