Feature Request: Enable multiuser support through a single port

Since Shadowsocks 2022 is a different protocol that many clients don't support, the standard Shadowsocks server should enable multiuser functionality through a single port. While Xray-core already has a similar feature, it's important for the official project to adopt this capability as well.

Xray-core configuration sample that support similar functionality:

{
    "inbounds": [
        {
            "port": 1234,
            "protocol": "shadowsocks",
            "settings": {
                "clients": [
                    {
                        "password": "example_user_1",
                        "method": "aes-128-gcm"
                    },
                    {
                        "password": "example_user_2",
                        "method": "aes-256-gcm"
                    },
                    {
                        "password": "example_user_3",
                        "method": "chacha20-poly1305"
                    }
                ],
                "network": "tcp,udp"
            }
        }
    ],
}

Start by reviewing the existing standard Shadowsocks server configuration and protocol handling, then compare them with the Xray-core configuration sample in the issue. Done means multiple clients with different passwords and encryption methods can share one port while supporting TCP and UDP.