adding sender vasp demo endpoints

[matthappens]

Sender VASP demo endpoints

this adds 2 demo endpoints

• receive an invoice from a receiver vasp and cache it for later client payment
• process a payrequest for a full incoming uma invoice, or a cached invoice

testing

run 2 vasps, a receiver vasp on 8080 and a sender vasp on 8081

receiver invoice to sender vasp
curl -u <username>:<password> -X POST "http://localhost:8081/api/uma/request_and_pay_invoice?invoice=<bech32 encoded invoice>"

sender client to sender vasp:
curl -u <username>:<password> -X POST "http://localhost:8081/api/uma/pay_invoice?invoice=<bech32 encoded invoice, or uuid identifying a cached invoice>

[matthappens]

• adding sender vasp demo endpoints #206 👈
• adding basic receiver vasp methods #205
• adding sending / receiving vasp methods for demo #204
• develop

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