create target ygot struct

evaluate yang models to see which ones we would like to support in our gnmi target as an optimal case

at least: interface, platform, system, acl - both the ubuntu and freebsd targets should work in their current state!

then create a singly ygot struct using the ygot generator that contains all the selected yang models, and refactor the gnmitarget (server, osclients) to use it

also figure out how to define which models are currently supported (capabilities), see modeldata.go (?)