Feat：SSE format

[zhuyanhuazhuyanhua]

Is your feature request related to a problem? Please describe.

I found that the splitting of the current SSE message is based on string matching, with data as the start and done as the end

Describe the solution you'd like

I want to use /n for splitting different fields, like id and event; /n/n is split as different SSE messages .SSE messages are cached with loop to ensure reliable transmission

Additional context

I have implemented SSE and LRU in Tencent Rhino Bird trpc-cpp and researched SSE best practices in detail
https://github.com/trpc-group/trpc-cpp/issues/207
https://html.spec.whatwg.org/multipage/server-sent-events.html
https://github.com/trpc-group/trpc-cpp/pull/225
If you think it is necessary, I am willing to complete this task

[zhuyanhuazhuyanhua]

struct SseEvent {
  std::string event_type;                    ///< Event type
  std::string data;                          ///< Event data
  std::optional<std::string> id;             ///< Event ID (optional) 
  std::optional<uint32_t> retry;             ///< Retry timeout in milliseconds (optional)

[zhuyanhuazhuyanhua]

/// @brief Serialize SSE event to string format
  std::string ToString() const {
    std::string result;

    if (!event_type.empty()) {
      result += "event: " + event_type + "\n";
    }

    if (!data.empty()) {
      // Handle multi-line data - split on both literal "\n" sequences and actual newlines
      size_t pos = 0;
      size_t prev_pos = 0;

      // First, handle literal "\n" sequences
      while ((pos = data.find("\\n", prev_pos)) != std::string::npos) {
        result += "data: " + data.substr(prev_pos, pos - prev_pos) + "\n";
        prev_pos = pos + 2; // Skip over "\n"
      }

      // Then handle any remaining actual newline characters
      std::string remaining = data.substr(prev_pos);
      if (!remaining.empty()) {
        size_t nl_pos = 0;
        size_t nl_prev_pos = 0;
        while ((nl_pos = remaining.find('\n', nl_prev_pos)) != std::string::npos) {
          result += "data: " + remaining.substr(nl_prev_pos, nl_pos - nl_prev_pos) + "\n";
          nl_prev_pos = nl_pos + 1; // Skip over the newline
        }
        // Add the last part
        if (nl_prev_pos < remaining.length()) {
          result += "data: " + remaining.substr(nl_prev_pos) + "\n";
        }
      }
    }

    if (id.has_value() && !id.value().empty()) {
      result += "id: " + id.value() + "\n";
    }

    if (retry.has_value()) {
      result += "retry: " + std::to_string(retry.value()) + "\n";
    }

    result += "\n";  // End with double newline
    return result;
  }
};

[zhuyanhuazhuyanhua]

done