Exports the given key into the specified format, if supported.
If the <CryptoKey> is not extractable, the returned promise will reject.
When format is either 'pkcs8' or 'spki' and the export is successful, the returned promise will be resolved with an <ArrayBuffer> containing the exported key data.
When format is 'jwk' and the export is successful, the returned promise will be resolved with a JavaScript object conforming to the JSON Web Key specification.