Add network socket example for ktor-network and netty

libmc-common/src/commonMain/kotlin/cn/rtast/libmc/network/SocketContext.kt

@@ -7,8 +7,14 @@
 
 package cn.rtast.libmc.network
 
+import cn.rtast.libmc.crypto.ProtocolContextBuilder
+
 public abstract class SocketContext {
     public abstract val engine: SocketEngine
 
     public fun createSocket(host: String, port: Int): RawSocket = engine.create(host, port)
-}
+}
+
+public fun (ProtocolContextBuilder.() -> Unit).withCustom(
+    block: ProtocolContextBuilder.() -> Unit,
+): ProtocolContextBuilder.() -> Unit = { this@withCustom.invoke(this); this.block() }