Add `amount_for_fees` to `EventPaymentSentSuccess`

[LefterisJP]

Problem Definition

As also seen from this issue when a payment is succesfully sent and has had fees paid the amount of the event is only the amount transferred without the fees.

That is really confusing. When the user sees this he think okay I spent 1000 tokens. But then his balance got reduced by 1012 tokens and he thinks a bug made his lose money!!!

Task

Add the amount_for_fees as a field of the EventPaymentSentSuccess. This way users who consume the event from the API will have all the information they need about the payment that happened and will not be surprised.

[RyanRussell00]

@ezdac Is this still an open issue?