strongswan: swanctl: Add support for encap

strongswan: swanctl: Add support for encap

Support the [encap] connection configuration option to force UDP
encapsulation of ESP packets to work around connectivity issues with
middleboxes which block ESP 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

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

Signed-off-by: Kevin Locke <redacted>