Port to Raspberry Pi Pico W core

Add support for the onboard WiFi chip on the Raspberry Pi Pico W
(RP2040 based ) board using the arduino-pico Arduino core at
https://github.com/earlephilhower/arduino-pico

The PicoW WiFi stack is a mashup of the ESP8266 and ESP32 cores, so
only minimal changes were required.  Defines a new NETWORK_TYPE for
the PicoW.

ESP8266 examples renames to ESP8266_PICO because they all work unmodified
(except for OTA which is handled differently on the Pico)

examples/esp8266_pico/WebSocketClientStomp/WebSocketClientStomp.ino

@@ -0,0 +1,149 @@
+/*
+    WebSocketClientStomp.ino
+
+    Example for connecting and maintining a connection with a STOMP websocket connection.
+    In this example, we connect to a Spring application (see https://docs.spring.io/spring/docs/current/spring-framework-reference/html/websocket.html).
+
+    Created on: 25.09.2017
+    Author: Martin Becker <mgbckr>, Contact: becker@informatik.uni-wuerzburg.de
+*/
+
+// PRE
+
+#define USE_SERIAL Serial
+
+
+// LIBRARIES
+
+#include <Arduino.h>
+#include <Hash.h>
+
+#include <ESP8266WiFi.h>
+#include <WebSocketsClient.h>
+
+
+// SETTINGS
+
+const char* wlan_ssid             = "yourssid";
+const char* wlan_password         = "somepassword";
+
+const char* ws_host               = "the.host.net";
+const int   ws_port               = 80;
+
+// URL for STOMP endpoint.
+// For the default config of Spring's STOMP support, the default URL is "/socketentry/websocket".
+const char* stompUrl            = "/socketentry/websocket"; // don't forget the leading "/" !!!
+
+
+// VARIABLES
+
+WebSocketsClient webSocket;
+
+
+// FUNCTIONS
+
+/**
+ * STOMP messages need to be NULL-terminated (i.e., \0 or \u0000).
+ * However, when we send a String or a char[] array without specifying 
+ * a length, the size of the message payload is derived by strlen() internally,
+ * thus dropping any NULL values appended to the "msg"-String.
+ * 
+ * To solve this, we first convert the String to a NULL terminated char[] array
+ * via "c_str" and set the length of the payload to include the NULL value.
+ */
+void sendMessage(String & msg) {
+    webSocket.sendTXT(msg.c_str(), msg.length() + 1);
+}
+
+void webSocketEvent(WStype_t type, uint8_t * payload, size_t length) {
+
+    switch (type) {
+        case WStype_DISCONNECTED:
+            USE_SERIAL.printf("[WSc] Disconnected!\n");
+            break;
+        case WStype_CONNECTED:
+            {
+                USE_SERIAL.printf("[WSc] Connected to url: %s\n",  payload);
+                
+                String msg = "CONNECT\r\naccept-version:1.1,1.0\r\nheart-beat:10000,10000\r\n\r\n";
+                sendMessage(msg);
+            }
+            break;
+        case WStype_TEXT:
+            {
+                // #####################
+                // handle STOMP protocol
+                // #####################
+
+                String text = (char*) payload;
+                USE_SERIAL.printf("[WSc] get text: %s\n", payload);
+
+                if (text.startsWith("CONNECTED")) {
+
+                    // subscribe to some channels
+
+                    String msg = "SUBSCRIBE\nid:sub-0\ndestination:/user/queue/messages\n\n";
+                    sendMessage(msg);
+                    delay(1000);
+
+                    // and send a message
+
+                    msg = "SEND\ndestination:/app/message\n\n{\"user\":\"esp\",\"message\":\"Hello!\"}";
+                    sendMessage(msg);
+                    delay(1000);
+                    
+                } else {
+
+                    // do something with messages
+                    
+                }
+
+                break;
+            }
+        case WStype_BIN:
+            USE_SERIAL.printf("[WSc] get binary length: %u\n", length);
+            hexdump(payload, length);
+
+            // send data to server
+            // webSocket.sendBIN(payload, length);
+            break;
+    }
+
+}
+
+void setup() {
+
+    // setup serial
+
+    // USE_SERIAL.begin(921600);
+    USE_SERIAL.begin(115200);
+
+    //    USE_SERIAL.setDebugOutput(true);
+
+    USE_SERIAL.println();
+
+
+    // connect to WiFi
+
+    USE_SERIAL.print("Logging into WLAN: "); Serial.print(wlan_ssid); Serial.print(" ...");
+    WiFi.mode(WIFI_STA);
+    WiFi.begin(wlan_ssid, wlan_password);
+
+    while (WiFi.status() != WL_CONNECTED) {
+        delay(500);
+        USE_SERIAL.print(".");
+    }
+    USE_SERIAL.println(" success.");
+    USE_SERIAL.print("IP: "); USE_SERIAL.println(WiFi.localIP());
+
+
+    // connect to websocket
+    webSocket.begin(ws_host, ws_port, stompUrl);
+    webSocket.setExtraHeaders(); // remove "Origin: file://" header because it breaks the connection with Spring's default websocket config
+    //    webSocket.setExtraHeaders("foo: I am so funny\r\nbar: not"); // some headers, in case you feel funny
+    webSocket.onEvent(webSocketEvent);
+}
+
+void loop() {
+    webSocket.loop();
+}