Lookup a certificate in the PKCS#11 storage by the given URI.
When the operation is finished, callback will be called. You can then call new_from_uri_async.end to get the result of the operation.
| pkcs11_uri |
a A PKCS #11 URI |
| cancellable | |
| callback |
a TaskReadyCallback to call when the operation completes |
| user_data |
the data to pass to callback function |