license name used for the API.
                * `identifier`: (`str`) An [SPDX](https://spdx.dev/) license expression
                    for the API. The `identifier` field is mutually exclusive of the `url`
                    field. Available since OpenAPI 3.1.0, FastAPI 0.99.0.
                * `url`: (`str`) A URL to the license used for the API. This MUST be
                    the format of a URL.