ipsec: support `leftsubnets` and `rightsubnets` options.

Instead of using `Vec<String>` for these two options, we use String in
order to align with what `ipsec.conf` manpage describes without any
parsing/modification.

Need NetworkManager-libreswan 1.2.31+.

Integration test case included.

Resolves: https://issues.redhat.com/browse/RHEL-32947

Signed-off-by: Gris Ge <fge@redhat.com>