POST /v2/transactions — index-linked-bond-trade-transaction

Buy or sell an index-linked bond. `indexFactor` is optional on the
trade itself; if omitted the system uses the index value for the
trade date.

Request:
{
  "data": {
    "type": "index-linked-bond-trade-transaction",
    "attributes": {
      "accountAmount":   "number (required) — cash impact, in account currency",
      "accruedInterest": "number (required) — interest since last repayment, in instrument currency",
      "commission":      "number (required) — bank/custodian fee, in account currency. Positive = paid",
      "indexFactor":     "string (optional) — index factor for the trade",
      "unitPrice":       "number (required) — price per unit",
      "units":           "number (required) — units bought (positive) or sold (negative)",
      "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)" } },
      "depository": { "data": { "type": "depository", "id": "string (required)" } }
    }
  }
}

Response (201 Created):
{
  "data": {
    "id": "string",
    "type": "index-linked-bond-trade-transaction",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}