server: split HTTP into its own interface

[ngxson]

Fix #16488

How it works:

sequenceDiagram
    participant User
    participant server_http_context
    participant server_http_res
    
    User->>server_http_context: request
    server_http_context->>server_http_req: create request
    server_http_req->>handler:
    handler->>server_http_res: create response
    
    loop for each result
        server_http_res->>server_http_context: response chunk
        server_http_context->>User: response chunk
        server_http_context->>server_http_res: next()
    end

    server_http_res->>server_http_context: terminate
    server_http_context->>User: close connection

Loading

• Each endpoint handler returns a server_res_generator, which is a derived class from server_http_res
• The server_res_generator indicates one of 2 modes: stream or non-stream
  • In non-stream mode, we simply return the data back to user
  • In stream mode, we call server_res_generator::next() until it returns false. Each time we call next(), we get a new chunk of data

TODO:

• fix error handling
• add exception handler at server_routes level

---

Testing:

• passed automated tests.sh
• test normal usage with web UI (with multimodal input)
• test usage with web UI, with concurrent requests and random interruptions

[ngxson]

No rush for reviewing this, would appreciate if you can do some testings on your side @ggerganov

In the next PR, I'll try to break the server.cpp into smaller pieces, the rough plan will be:

• server-context.cpp
• server-queue.cpp
• server-task.cpp (containing both task + response + queue)
• server-common.cpp (everything else)

---

While working on this, I'm also thinking about maybe re-using server code in llama-cli (I made a demo here); the main benefit will be to bring the same webui experience into CLI, including multimodal support, conversation control (delete/regenerate message), tool call, etc. The old CLI can be moved to llama-completion and the chat support will be removed from it. What do you think about this idea?