Suggestion: Add FRP client support

FRP is a robust reverse tunnel program that I have used for many years on my system. It includes a P2P method called XTCP, which can establish a reverse tunnel even behind NAT. In the context of the GFW, inbound requests are permitted, so a reverse tunnel can help bypass GFW restrictions. If possible, please add integration of FRP to sing-box.

I tested this on my home internet in Iran, and it works great, but it is difficult to set up separately.

https://github.com/fatedier/frp

example :

  "outbounds": [
    {
      "tag": "vmess-out",
      "type": "vmess",
      "server": "::1",
      "server_port": 8080,
      "uuid": "xxxxxx",
      "frpc": {
        "enable": true,
        "serverAddr": "x.x.x.x",
        "serverPort": 8443,
        "auth": {
          "method": "token",
          "token": "yyyyyy"
        },
        "transport": {
          "protocol": "wss",
          "tcpMux": true,
          "poolCount": 5,
          "tls": {
            "enable": true,
            "certFile": "cert.crt",
            "keyFile": "key.pem",
            "trustedCaFile": "ca.crt",
            "serverName": "test.com"
          }
        },
        "visitors": [
          {
            "name": "vmess-reverse-p2p",
            "type": "xtcp",
            "serverName": "vmess-p2p",
            "secretKey": "zzzzzz",
            "bindAddr": "::1",
            "bindPort": 8080,
            "keepTunnelOpen": true
          }
        ]
      }
    }
  ]

No sing-box file, test, or entry point is named. Start by reviewing the linked FRP project and its XTCP client behavior, then determine how the proposed outbound configuration maps to sing-box; done would mean documented, working FRP integration with the demonstrated reverse-tunnel options.