SPGW to use openflow

[spencersevilla]

Since forking the SPGW, Eurecom rearchitected the code to provide support for OpenFlow as well as kernelspace GTPNL. True to form, the current OPENAIRINTERFACE/openair-cn master and develop branches fail to compile if you enable openflow support.

Given that our SPGW runs perfectly, I specifically did not want to track their code refactoring, because we actually run a production network and I don't want to break things. However, OpenFlow support would be fantastic to have, for several reasons, primarily that it would let us move the SPGW out of the network data plane. Let's look at their code base, take what we can, fix what we have to, and create an SPGW that actually supports OpenFlow 👍