asio: initial idf port of asio library without ssl

* Original commit: espressif/esp-idf@1ef13c524c

examples/protocols/asio/udp_echo_server/main/Kconfig.projbuild

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+menu "Example Configuration"
+
+config ESP_WIFI_SSID
+    string "WiFi SSID"
+    default "myssid"
+    help
+	SSID (network name) for the example to connect to.
+
+config ESP_WIFI_PASSWORD
+    string "WiFi Password"
+    default "mypassword"
+    help
+	WiFi password (WPA or WPA2) for the example to use.
+
+config EXAMPLE_PORT
+    string "asio example port number"
+    default "80"
+    help
+    Port number used by ASIO example
+
+endmenu

examples/protocols/asio/udp_echo_server/main/component.mk

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+#
+# Main component makefile.
+#
+# This Makefile can be left empty. By default, it will take the sources in the 
+# src/ directory, compile them and link them into lib(subdirectory_name).a 
+# in the build directory. This behaviour is entirely configurable,
+# please read the ESP-IDF documents if you need to do this.
+#

examples/protocols/asio/udp_echo_server/main/udp_echo_server.cpp

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+//
+// async_udp_echo_server.cpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2018 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <cstdlib>
+#include <iostream>
+
+#include "asio.hpp"
+
+using asio::ip::udp;
+
+class server
+{
+public:
+  server(asio::io_context& io_context, short port)
+    : socket_(io_context, udp::endpoint(udp::v4(), port))
+  {
+    do_receive();
+  }
+
+  void do_receive()
+  {
+    socket_.async_receive_from(
+        asio::buffer(data_, max_length), sender_endpoint_,
+        [this](std::error_code ec, std::size_t bytes_recvd)
+        {
+          if (!ec && bytes_recvd > 0)
+          {
+            std::cout << data_ << std::endl;
+            do_send(bytes_recvd);
+          }
+          else
+          {
+            do_receive();
+          }
+        });
+  }
+
+  void do_send(std::size_t length)
+  {
+    socket_.async_send_to(
+        asio::buffer(data_, length), sender_endpoint_,
+        [this](std::error_code /*ec*/, std::size_t  bytes /*bytes_sent*/)
+        {
+          do_receive();
+        });
+  }
+
+private:
+  udp::socket socket_;
+  udp::endpoint sender_endpoint_;
+  enum { max_length = 1024 };
+  char data_[max_length];
+};
+
+void asio_main()
+{
+    asio::io_context io_context;
+
+    server s(io_context, std::atoi(CONFIG_EXAMPLE_PORT));
+
+    io_context.run();
+}