Add support for setting PDU Session Container Type to UL

[linouxis9]

Add support for setting PDU Session Container Type to UL in the GTP-U Extension Header.

This is required for PacketRusher and I would like to avoid using a gtp5g fork inside PacketRusher, I think it's better if this kind of work can be upstreamed :-)

I stuffed the field in FAR as I think it was the better fit, but if you have any suggestions on a better place, please say so. The location of the field is a breaking change as well, so we might need to move that.

The related go-gtp5nl PR: https://github.com/free5gc/go-gtp5gnl/pull/23

Thanks a lot! Valentin