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

Coupon payment on an index-linked bond. `indexFactor` scales the
coupon for inflation accrual.

Request:
{
  "data": {
    "type": "index-linked-bond-coupon-transaction",
    "attributes": {
      "accountAmount": "number (required) — amount deposited/withdrawn, in account currency",
      "grossAmount":   "number (required) — total coupon amount, in instrument currency",
      "indexFactor":   "number (required) — index factor applied for the date",
      "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-coupon-transaction",
    "attributes": { /* same as request */ },
    "relationships": { /* same as request */ }
  }
}