Create Rust-based openai api proxy server in node hub

[phil-opp]

This is a Rust-based version of #676 based on https://github.com/LlamaEdge/LlamaEdge/tree/main/llama-api-server . It includes serde structs to fully deserialize chat completion requests.

The server replies to requests asynchronously, which makes it possible to serve multiple clients at the same time. Note that replies are mapped to requests purely based on their order right now. So the first reply is assigned to the first request and so on. If the order of replies is different (e.g. because some completions take longer), some form of ID needs to be added to the code to enable a proper request<->reply mapping.

Like #676, this PR only implements basic chat completion requests. You can try it through these steps:

cd examples/openai-server
dora build dataflow-rust.yml
dora start dataflow-rust.yml

# In a separate terminal
python openai_api_client.py

I didn't implement the /v1/models endpoint, so the "Testing API endpoints..." part of openai_api_client.py is expected to fail with "Error listing models: 404".

[phil-opp]

We discussed that we want to merge this directly to enable preliminary testing.