Record the sender in incomingPayments

[adrianhopebailie]

As the receiver I would usually expect to find something in the externalRef that would help me correlate the payment to some external process or event (an invoice number, order number or similar).

For the P2P use case it would be useful to have some record in the incoming payment of who sent the payment.

We could either:

1. Use the externalRef field but risk this being used incorrectly (i.e. we can't guarantee that it will hold a payment pointer when the use case is a P2P payment)
2. Add a new field that MUST be a payment pointer (if it is used).