luci-proto-wireguard: fix generated AllowedIPs

This corrects the option `AllowedIPs` in generated peer configurations,
and allows to customize it via a dropdown list.

Fixes: #5956
Signed-off-by: Julien Cassette <julien.cassette@gmail.com>
[correct fixes tag, slightly adjust option description]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>