POST /v2/transactions — futures-execution-transaction

The settlement / cash-flow leg of a futures contract — links back to
the opening futures-trade-transaction via `openingTransaction`.

Request:
{
  "data": {
    "type": "futures-execution-transaction",
    "attributes": {
      "accountAmount": "number (required) — cash impact, in account currency",
      "commission":    "number (required) — bank/custodian fee, in account currency. Positive = paid",
      "unitPrice":     "number (required) — execution price per unit",
      "units":         "number (required) — units settled",
      "comment":       "string (optional)",
      "tradeDate":     "string, ISO 8601 (required) — trade date, end-of-day",
      "valueDate":     "string, ISO 8601 (required) — value date, end-of-day"
    },
    "relationships": {
      "account":             { "data": { "type": "account", "id": "string (required)" } },
      "instrument":          { "data": { "type": "instrument", "id": "string (required)" } },
      "openingTransaction":  { "data": { "type": "futures-trade-transaction", "id": "string (required)" } },
      "depository":          { "data": { "type": "depository", "id": "string (required)" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "futures-execution-transaction",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}