[FEATURE] Include payout_id on the Refund schema

[ghostwalkerj]

Is your feature request related to a problem? Please describe.
When creating a refund using the API, there is no way to determine the related payout object. Therefore, can't create an invoice, a refund and issue a payout purely via the API, as the payout id is not returned, even though it is saved in the refund table.

Describe the solution you'd like
Consider adding an optional payout_id string to the refund schema.

Additional context
Without this feature, integrating Bitcart fully via the API is not possible.

[ghostwalkerj]

Related PR #373

[MrNaif2018]

Merged! Thanks