Add support for server-side threads.

[drazvan]

This PR adds basic support for storing threads on the server side. This is useful when the calls to the guardrails server come from an integration (e.g., with a chat plugin), and it's not straightforward to send the entire conversation history.

This is enabled by allowing an extra thread_id to be specified in the body of the requests. Check out the documentation for detailed usage, configuration and limitations.

POST /v1/chat/completions

{
    "config_id": "config_1",
    "thread_id": "1234567890123456",
    "messages": [{
      "role":"user",
      "content":"..."
    }]
}

[trebedea]

Looks good 👍