Support for Ceres networks in onnx backend

[borg323]

The Ceres networks are in onnx format so we would like to get them running with the lc0 onnx backend. As far as I know, the input format is a bit different but the outputs are compatible, and now with the new backend interface supporting them is straightforward.

I expect the following 3 steps should be enough to get it to work:

1. Port the onnx backend to the new interface. This should be relatively easy, reusing code from the wrapper where needed.
2. Define the Ceres input format in net.proto
3. Make EncodePositionForNN() support the Ceres input format.