Feature - Payment request

[weilbith]

Description

During the EthCC convention I realized the following after a couple of interesting talks: it is not always affordable/nice for a user to open a direct channel with a service provider he wants to pay. If you only need to do payments to the them once in a while or just on a not regular basis, it is better to just make a mediated transfer in the network. Here it would be nice if the service provider client could offer the user client to pay for the path finding service to find a path in the network. A solution therefore would be that a client can make a PFS request with himself as target. Then this client can send a payment request message to the sender of the intended payment (the first node in the path). This client probably should ask the user to confirm the payment. This capability might be only enabled for end-user clients which can interact with the client. And full-automated payment request acceptance could be configurable, maybe even with a while list. But I'm unsure how useful that actually is.
The feature shouldn't be hard to add (I hope the PFS interface already accepts explicit sender and target address parameters). But it could be very useful for applications using Raiden.

Acceptance criteria

Tasks

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