Merge `raft_proto` into `nvforest` namespace

[hcho3]

The raft_proto namespace contains many components of nvForest's public API. For example, the signature of the predict() function is as follows:

void predict(raft_proto::handle_t const& handle,
               io_t* output,
               io_t* input,
               std::size_t num_rows,
               raft_proto::device_type out_mem_type,
               raft_proto::device_type in_mem_type,
               infer_kind predict_type                        = infer_kind::default_kind,
               std::optional<index_type> specified_chunk_size = std::nullopt)

where raft_proto::device_type and raft_proto::handle_t are really part of the public API of nvForest. My suggestion is to simply merge raft_proto into the nvforest namespace.

[wphicks]

I'm really happy to see this! This is how I wanted things to be in the first place, and making the change now will make for a much more polished API now that nvforest is a separate library.