Mojaloop: Add support for "refund" API

[edcable]

In order to be compliant with the L1 principles, we should enable DFSPs to providing refunds by supporting the approach mandated by the API: https://github.com/mojaloop/mojaloop-specification/blob/master/documents/Use%20Cases.md#411-refund

From which Istvan inquired,
hi Sam, previously I have found this info on Refunds. https://mojaloop.io/documentation/contributors-guide/frequently-asked-questions.html#35-why-are-there-no-reversals-allowed-on-a-mojaloop (seems not accessible on the new site)
can you share some info, where we could find the links to the API related to refund?

Am I right, that executing the refund is just a normal payment, with references to the original transaction from the payee side of the original transaction?

Am I right, that there is no API to send a refund request from the payer side?

To which, Sam replied:

To your question - From the Payer side, there is no request or resource in the API that can be used to raise this request directly, but it can be raised as a request indirectly on /transactionRequests. Co-ordination of this has to be done by other channels. This is discussed in the API Definition indicating that it should be raised by a Customer Care Executive (CCE) of the initial Payer FSP by raising it with the Payee FSPs representatives or using a similar mechanism.

Otherwise its similar to a normal payment (Payee initiated using /transactionRequests), and there are fields for capturing information specific to Refunds, for example in transactionType, which has a more specific refundInfo to capture the refund related information and such other fields