POST /v3/instruments — index-linked-bond-instrument

Create an inflation-linked bond instrument. Schema mirrors a regular
bond — the inflation indexing is applied via index-factor time-series
data and via the `indexFactor` attribute on coupon / redemption /
trade transactions.

Request:
{
  "data": {
    "type": "index-linked-bond-instrument",
    "attributes": {
      "ccyCode":            "string (required) — ISO 4217",
      "countryCode":        "string (required) — ISO 3166-1 alpha-2 issuer domicile",
      "couponFrequency":    "string (required) — quarterly | semiannually | annually",
      "datedDate":          "string, ISO 8601 (optional) — start date for accrued interest",
      "dayCountConvention": "string (required) — actual/actual | 30/360 | actual/360",
      "firstCouponDate":    "string, ISO 8601 (optional)",
      "maturityDate":       "string, ISO 8601 (optional)",
      "name":               "string (required) — display name",
      "securityIdentifier": "string (required) — external security identifier"
    }
  }
}

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

Note: existing instruments may have `couponFrequency` or
`dayCountConvention` set to "other"; that value cannot be supplied on
create but can be returned on read.