Treat rates as composite exchange and network rates

[sabineschaller]

Context

Currently, Rafiki requires an exchange rates endpoint EXCHANGE_RATES_URL, that provides exchange rates in the format

{
  base: "CURRENCY_CODE_0",
  rates: {
    "CURRENCY_CODE_1": value, // exchange rate
    ...
  }
}

However, as soon as we enable multi-hop routing, the rates should not just reflect the exchange rates but also the fee of forwarding packets. No middle connector would want to forward the packets for free. Hence, we need to update the endpoint to be called RATES_URL again and to change the expected response to be of the form

{
  base: "CURRENCY_CODE_0",
  rates: {
    "CURRENCY_CODE_0": value, // just forwarding fees
    "CURRENCY_CODE_1": value,  // exchange rate + exchange fee + forwarding fee
    ...
  }
}

Todos

• [ ] rename EXCHANGE_RATES_URL to RATES_URL
• [ ] rename EXCHANGE_RATES_LIFETIME to RATES_LIFETIME
• [ ] rename and update OpenAPI spec
• [ ] update docs to explain expected behavior