ChangeNOW exchange API.

  GET  /v3/exchange/estimate   quote an exchange
  POST /v3/transactions        create the order
  GET  /v3/transactions/{id}   read the status of an order

Every endpoint carries its own guide: add help to a GET, as in
GET /v3/exchange/estimate?help, or help=true to the body of a POST. Nothing else
in the request is read when help is asked for.

Identifiers come from GET /v1/assets. Pass the canonical ticker and its network
separately: usdt with trx, not usdttrc20. A legacy ticker is accepted and
answered in canonical form.

Errors are returned as {"status", "errors", "help"}. Every entry in errors is
keyed by the parameter at fault and carries a machine code, an English message,
the expected value and, where the set is finite, the allowed values. 4xx means
the request has to change, 5xx means it does not and a retry is the right move.

Full specification: https://changenow.io/openapi.json