Add Edge TPU support to C++ engine

[tommadams]

The TFLite C++ runtime now supports the Coral Edge TPU: https://coral.ai/docs/edgetpu/tflite-cpp/

We should hook that up.

[aki65]

Speaking of the Edge TPU, you published a minigo-tflite-file compiled for the Edge TPU about a year ago which offered very interesting possibilities. Could you perhaps publish the original quantized tflite-file too (i.e. before compiling for the Edge TPU). This would make it much easier to compare results to the non-quantized version und evaluate quantization effects.