openvpn: Add option to download a client package with PEM files

openvpn: Add option to download a client package with PEM files

This patch adds the option to download a client package
that comes with a regular PEM and key file instead of a
PKCS12 file which is easier to use with clients that
don't support PKCS12 (like iOS) opposed to converting
the file manually.

This requires that the connection is created without
using a password for the certificate. Then the certificate
is already stored in an insecure way.

This patch also adds this to the Core Update 95 updater.

Fixes: #10966
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
CC: Alexander Marx <alexander.marx@ipfire.org>