Loopback outbound: Add sniffing (#6326)

Example: https://github.com/XTLS/Xray-core/pull/6326#issue-4659701786

proxy/loopback/config.proto

@@ -6,6 +6,9 @@ option go_package = "github.com/xtls/xray-core/proxy/loopback";
 option java_package = "com.xray.proxy.loopback";
 option java_multiple_files = true;
 
+import "app/proxyman/config.proto";
+
 message Config {
   string inbound_tag = 1;
+  xray.app.proxyman.SniffingConfig sniffing = 2;
 }