Support for managing payment fees

Please add support for the following endpoints:

[`/v2/payments/payment_fees`](https://developer.currencycloud.com/api-reference/#get-payment-fees)
[`/v2/payments/assign_payment_fee`](https://developer.currencycloud.com/api-reference/#assign-payment-fee)
[`/v2/payments/unassign_payment_fee`](https://developer.currencycloud.com/api-reference/#unassign-payment-fee)