Missing FOU and GUE tunneling protocols

Hi,

Recent OpenWRT kernels have FOU and GUE support. However in the official documentation about tunnelling protocols they are missing: https://openwrt.org/docs/guide-user/network/tunneling_interface_protocols

This is because these protocols are recently added. So, it will be interesting to have support for them in the LUCI interface.

If you need more info about them read: http://man7.org/linux/man-pages/man8/ip-gue.8.html However, think you on these protocols as the best replacements for "gretap" and "eoip"/"ipip"/etc. The transport layer is pure UDP so it's easy to manage. And it can encapsulate anything; so L2 & L3 tunnels are possible using a simple UDP flow.

I'm using both manually, so I hope someone think on add support for them in LUCI. Regards.