UPSTREAM PR #17174: server: (refactor) implement generator-based API for task results

[DajanaV]

Mirrored from ggml-org/llama.cpp#17174

This PR adds a generator-based API for receiving task results. It aims to reduce the usage of callback function, making the code looks more "linear", easier to follow.

This also allowing to return correct HTTP error code in streaming case, ref: ggml-org/llama.cpp#16486 (comment)

Example:

server_response_generator gen(ctx_server);
{
    std::vector<server_task> tasks;
    // ... populate tasks ...
    gen.post_tasks(std::move(tasks));
}

// wait for the results
auto all_results = gen.wait_for_all(req.is_connection_closed);

// collect results
if (all_results.is_terminated) {
    return; // connection is closed
} else if (all_results.error) {
    res_error(res, all_results.error->to_json());
    return;
} else {
    for (auto & res : all_results.results) {
        GGML_ASSERT(dynamic_cast<server_task_result_embd*>(res.get()) != nullptr);
        responses.push_back(res->to_json());
    }
}