Add NetworkSocket examples (#80)

Author: mprse

APIs_NetworkSocket/CellularNonIPSocket/README.md

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+# CellularNonIPSocket example
+
+This example shows how to create and use a cellular non-IP socket.

APIs_NetworkSocket/CellularNonIPSocket/main.cpp

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+/*
+ * Copyright (c) 2006-2020 Arm Limited and affiliates.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "mbed.h"
+#include "CellularNonIPSocket.h"
+#include "CellularDevice.h"
+
+// Network interface
+NetworkInterface *iface;
+
+int main()
+{
+    // Bring up the cellular interface
+    iface = CellularContext::get_default_nonip_instance();
+    MBED_ASSERT(iface);
+
+    // sim pin, apn, credentials and possible plmn are taken automatically from json when using NetworkInterface::set_default_parameters()
+    iface->set_default_parameters();
+
+    printf("Cellular Non-IP Socket example\n");
+    if (NSAPI_ERROR_OK != iface->connect() || NSAPI_STATUS_GLOBAL_UP != iface->get_connection_status()) {
+        printf("Error connecting\n");
+        return -1;
+    }
+
+    CellularNonIPSocket sock;
+
+    nsapi_error_t retcode = sock.open((CellularContext *)iface);
+
+    if (retcode != NSAPI_ERROR_OK) {
+        printf("CellularNonIPSocket.open() fails, code: %d\n", retcode);
+        return -1;
+    }
+
+    const char *send_string = "TEST";
+
+    if (0 > sock.send((void *) send_string, sizeof(send_string))) {
+        printf("Error sending data\n");
+        return -1;
+    }
+
+    printf("Success sending data\n");
+
+    char recv_buf[4];
+    if (0 > sock.recv((void *)recv_buf, sizeof(recv_buf))) {
+        printf("Error receiving data\n");
+        return -1;
+    }
+
+    printf("Success receiving data\n");
+
+    // Close the socket and bring down the network interface
+    sock.close();
+    iface->disconnect();
+    return 0;
+}