Transfer certificate data of X.509 encryption certificate to PIV cards

* Add WriteCertAssuanTransaction implementing the inquire() callback
for sending the certificate data to scdaemon.
* Add startTransaction() to ReaderStatus for starting a transaction using
a custom AssuanTransaction subclass.
* Make KeyToCardCommand transfer the certificate data additionally to the
secret key data of X.509 encryption certificates to PIV cards.

GnuPG-bug-id: 4794