Add ability to use Indy Payments API

[seniorjoinu]

Detailed Description

Indy itself provides a way to abstract different kinds of payments and asset exchange.

Context

This would be great if Cordentity will also provide this feature.

Possible Implementation

https://github.com/hyperledger/indy-sdk/tree/master/wrappers/java/src/test/java/org/hyperledger/indy/sdk/payment
https://github.com/hyperledger/indy-sdk/tree/master/doc/design/004-payment-interface

[seniorjoinu]

We need to develop and specify an architecture for this feature.