Support sequoia-keystore server

Currently there is no way to use sequoia-pgp with for instance the openpgp-card keystore backend. It will always use the gpg-agent and in turn, scdaemon.

Considering how scdaemon does not like it very much when other applications are using the card this is not optimal, and a mode to use native backends without having to resort to the gpg agent would be much appreciated.