C/Other Languages: Support serialization and deserialization of Tensors #6003

In C++ and Python, Tensors can be serialized into a TensorProto using functions like Tensor::AsProtoTensorContents and make_tensor_proto

This is particularly useful when communicating tensors across process boundaries such as PredictRequest RPCs to a TensorFlow Model Server

Filing this to track any changes to the C API and language bindings.

Interested parties: @asimshankar @jhseu @nfiedel @josh11b