Enable signing via provider for management-external-key

Enable signing via provider for management-external-key

- Add a function to set as sign_op during key import. The
  function passes the signature request to management interface,
  and returns the result to the provider.

v2 changes: Method to do digest added to match the changes in
            the provider signature callback.
TODO:
 - Allow passing the undigested message to management interface
 - Add pkcs1 DigestInfo header when required

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-8-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23428.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>