strongswan: swanctl: Add support for send_certreq

strongswan: swanctl: Add support for send_certreq

Support the [send_certreq] connection configuration option to disable
offering trusted root CA certificates and reduce the size of the initial
IKE packets.

This work is based on a patch by @aleks-mariusz in
https://forum.openwrt.org/t/confusion-regarding-setting-up-ikev2-vpn-service-with-strongswan-using-ipsec-and-swanctl/169587/9

[send_certreq]: https://docs.strongswan.org/docs/latest/swanctl/swanctlConf.html#_connections

Signed-off-by: Kevin Locke <redacted>