Add "echo data" in the quote response and transfer request

[adrianhopebailie]

If #13 is implemented to align the API with ILPv4 it will be necessary to define a data element in the callback of the /quote API to carry data sent by the payee that it expects to be unchanged and returned in the subsequent /transfer API call. (Currently payee systems will use the encoded ILP packet)

This field will be used by the payee to generate the fulfilment and condition so it must be returned unchanged to the payee in order to fulfil the transfer. To satisfy this requirement it is recommended that the field is base64 encoded.

The content of the field is at the discretion of the payee and MAY contain data (such as the quote and transaction details) that it can use to ensure the fulfilment and condition are cryptographically bound to the transaction.