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.