Payments endpoints

[koladev32]

The Transfa API provides endpoints for payments where the client can send requests for listing, retrieving, creating, and refunding payments.
The package should have support for these endpoints.

Then, the package should allow the user to:

• create a payment request.
• list all payments for an organization.
• retrieve payment for an organization.
• refund a successful and completed payment.

Note that a payment request requires the user to provide an idempotency key in the headers. The package should automatically generate the idempotency key.