New SSL implementation

Added support to secured connexion to mqtt server thanks to WiFiClientSecure class.

Please see comments in code. You can look for WiFiClientSecure, MY_GATEWAY_ESP8266_SECURE, MY_SSL_CERT, MY_SSL_FINGERPRINT and MY_SSL_CERT_CLIENT in the code below to see what has changed. No new method, no new class to be used by my_sensors. 

The following constants have to be defined from the gateway code:
MY_GATEWAY_ESP8266_SECURE    in place of MY_GATEWAY_ESP8266 to go to secure connections.
MY_SSL_CERT_AUTHx            Up to three root Certificates Authorities could be defined to validate the mqtt server' certificate. The most secure. Let's Encrypt requires at least two to validate all the certificates signed by them.
MY_SSL_FINGERPRINT           Alternatively, the mqtt server' certificate finger print could be used. Less secure and less convenient as you'll have to update the fingerprint each time the mqtt server' certificate is updated If neither MY_SSL_CERT_AUTH1 nor MY_SSL_FINGERPRINT are defined, insecure connexion will be established. The mqtt server' certificate will not be validated.
MY_SSL_CERT_CLIENT           The mqtt server may require client certificate for 
MY_SSL_KEY_CLIENT            authentication.

Author: Tico06

MyConfig.h

@@ -1548,29 +1548,64 @@
 //#define MY_MQTT_SUBSCRIBE_TOPIC_PREFIX "mygateway1-in"
 
 /**
- * @def MY_MQTT_CA_CERT
- * @brief Set a specific CA certificate needed to validate MQTT server against. Use the certificate as a trust anchor, accepting remote certificates signed by it.
+ * @def MY_SSL_CERT_AUTHx
+ * @brief Up to three root Certificates Authorities could be defined to validate the mqtt server' certificate. The most secure.
+ *
+ * This define is mandatory when you need connect MQTT over SSL/TLS. Certificate Authorities.
+ * The best method to validate server certificates. 
+ * Advised to retrieve root Certificate Authorities as they expire less often than server certificates.
+ * With let's encrypt you may need up to three Certificate Authorities
  *
- * This define is mandatory when you need connect MQTT over SSL/TLS.
  * Example: @code
  *
- * const char mqtt_ca_cert[] PROGMEM = R"EOF(
+ * const char cert_isrgrootx1_Authority[] PROGMEM = R"EOF(
+ * ----- BEGIN THE CERTIFICATE -----
+ * XXX ... XXX
+ * ----- FINISH CERTIFICATE -----
+ * )EOF";
+ * 
+ * const char cert_isrgrootx2_Authority[] PROGMEM = R"EOF(
  * ----- BEGIN THE CERTIFICATE -----
  * XXX ... XXX
  * ----- FINISH CERTIFICATE -----
  * )EOF";
  *
- * #define MY_MQTT_CA_CERT mqtt_ca_cert
+ * const char cert_letsEncryptR3_Authority[] PROGMEM = R"EOF(
+ * ----- BEGIN THE CERTIFICATE -----
+ * XXX ... XXX
+ * ----- FINISH CERTIFICATE -----
+ * )EOF";
  *
+ * #define MY_SSL_CERT_AUTH1 cert_isrgrootx1_Authority
+ * #define MY_SSL_CERT_AUTH1 cert_isrgrootx2_Authority
+ * #define MY_SSL_CERT_AUTH1 cert_letsEncryptR3_Authority
+ *
+ * @endcode
+ */
+//#define MY_SSL_CERT_AUTH1
+
+/**
+ * @def MY_SSL_FINGERPRINT
+ * @brief Server certificate validation with its fingerprint
+ *
+ * The finger print to validate the mqtt server certificate. This is less secure and less convenient
+ * than using certificate authorities.
+ * Command (3 lines...) to obtain the certificate finger print:
+ * $>openssl s_client -connect <hostname>:<host port> < /dev/null 2>/dev/null | \
+ *           openssl x509 -fingerprint -noout -in /dev/stdin \
+ *           awk -F= '{print $2}'
+ *
+ * Example: @code
+ * const char mqtt_fingerprint [] PROGMEM = "CA:CE:2B:ED:D3:32:A7:F1:8C:73:9E:9B:B7:D5:75:4A:10:61:E4:05";
  * @endcode
  */
-//#define MY_MQTT_CA_CERT
+//#define MY_SSL_FINGERPRINT
 
 /**
- * @def MY_MQTT_CLIENT_CERT
+ * @def MY_SSL_CERT_CLIENT
  * @brief Set a client certificate to send to a MQTT server that requests one over TLS connection.
  *
- * This define is mandatory when you need connect MQTT over SSL/TLS.
+ * This define is mandatory when you need connect MQTT over SSL/TLS and client certificate is requested.
  * Example: @code
  *
  * const char mqtt_client_cert[] PROGMEM = R"EOF(
@@ -1579,17 +1614,17 @@
  * ----- FINISH CERTIFICATE -----
  * )EOF";
  *
- * #define MY_MQTT_CLIENT_CERT mqtt_client_cert
+ * #define MY_SSL_CERT_CLIENT mqtt_client_cert
  *
  * @endcode
  */
-//#define MY_MQTT_CLIENT_CERT
+//#define MY_SSL_CERT_CLIENT
 
 /**
- * @def MY_MQTT_CLIENT_KEY
- * @brief Set a client private key to send to a MQTT server that requests one over TLS connection.
+ * @def MY_SSL_KEY_CLIENT
+ * @brief Set the client private key generated with the MY_SSL_CERT_CLIENT.
  *
- * This define is mandatory when you need connect MQTT over SSL/TLS.
+ * This define is mandatory when you need connect MQTT over SSL/TLS and client certificate is requested.
  * Example: @code
  *
  * const char mqtt_client_key[] PROGMEM = R"EOF(
@@ -1598,11 +1633,11 @@
  * ----- FINISH THE RSA PRIVATE KEY -----
  * )EOF";
  *
- * #define MY_MQTT_CLIENT_KEY mqtt_client_key
+ * #define MY_SSL_KEY_CLIENT mqtt_client_key
  *
  * @endcode
  */
-//#define MY_MQTT_CLIENT_KEY
+//#define MY_SSL_KEY_CLIENT
 
 /**
  * @def MY_IP_ADDRESS
@@ -2373,9 +2408,12 @@
 #define MY_MQTT_CLIENT_ID
 #define MY_MQTT_PUBLISH_TOPIC_PREFIX
 #define MY_MQTT_SUBSCRIBE_TOPIC_PREFIX
-#define MY_MQTT_CA_CERT
-#define MY_MQTT_CLIENT_CERT
-#define MY_MQTT_CLIENT_KEY
+#define MY_SSL_CERT_AUTH1
+#define MY_SSL_CERT_AUTH2
+#define MY_SSL_CERT_AUTH3
+#define MY_SSL_FINGERPRINT
+#define MY_SSL_CERT_CLIENT
+#define MY_SSL_KEY_CLIENT
 #define MY_SIGNAL_REPORT_ENABLED
 // general
 #define MY_WITH_LEDS_BLINKING_INVERSE