Feature: decode an offer and then generate an invoice_request and return it over the RPC, not onion messaging

[AndySchroder]

I'd like to have the ability to generate an offer (coming in #170) and decode an offer (we have this with the existing decode-offer RPC) and then generate an invoice_request and return the generated invoice_request over the RPC and don't send it over onion messaging (what I'm asking for in this issue). This is in compliment to my request in #170 (comment) where I'd like to be able to receive the invoice_request through the RPC (in addition to receiving normally via onion messaging).

The goal is for the sender and receiver to be able to exchange an offer, invoice_request, and invoice over a local physical link in addition to onion messaging.

The node should wait for an invoice to be received back. If the invoice is received back over a local physical link (outside of lndk), the existing pay-invoice RPC can be used to pay the invoice. Otherwise, if the invoice is received back over onion messaging, the lndk node should automatically pay it.

Possibly some of the logic I'm asking for here is already in the get-invoice RPC?