Manage partial refund

[j4asper]

Create and pay for order to have an order in completed state, get the id.
Refund order with minimal required parameters (amount), provide partial amount, with the Sandbox Secret API key in the request header.

EXPECTED RESULT:
New order with type: refund is created.
Refund order's status is completed.
Refund transaction status is Completed in your Revolut Merchant account.