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

examples/GatewayESP8266SecureMQTTClient/certs.h

@@ -0,0 +1,133 @@
+#pragma once
+
+// The mqqt host and port
+const char* mqtt_host = "<mqtt_host>";
+const uint16_t mqtt_port = <mqtt_port>;
+
+//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}'
+const char mqtt_fingerprint [] PROGMEM = "CA:EE:2B:ED:D3:23:A7:F1:8C:73:9E:9B:B7:D5:75:41:10:61:E4:05";
+
+//Certificate Authorities. The best method to validate server certificates
+// Advised to retrieve root Certificate Authorities as they expire less often
+// than server certificates. Here after letsencrypt Certificate Authorities are listed.
+// They are available at https://letsencrypt.org/certificates/
+
+// Root Certificate Authorities ISRG Root X1
+// https://letsencrypt.org/certs/isrgrootx1.pem
+// Not Before: Jun  4 11:04:38 2015 GMT
+// Not After : Jun  4 11:04:38 2035 GMT
+const char cert_isrgrootx1_Authority [] PROGMEM = R"CERT(
+-----BEGIN CERTIFICATE-----
+MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw
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+emyPxgcYxn/eR44/KJ4EBs+lVDR3veyJm+kXQ99b21/+jh5Xos1AnX5iItreGCc=
+-----END CERTIFICATE-----
+)CERT";
+
+// Root Certificate Authorities ISRG Root X2
+// https://letsencrypt.org/certs/isrg-root-x2.pem
+// Not Before: Sep  4 00:00:00 2020 GMT
+// Not After : Sep 17 16:00:00 2040 GMT
+const char cert_isrgrootx2_Authority [] PROGMEM = R"CERT(
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+)CERT";
+
+// This one shouldn't be needed....
+// Root Certificate Authorities Let’s Encrypt R3
+// https://letsencrypt.org/certs/lets-encrypt-r3.pem
+// Not Before: Sep  4 00:00:00 2020 GMT
+// Not After : Sep 15 16:00:00 2025 GMT
+const char cert_letsEncryptR3_Authority [] PROGMEM = R"CERT(
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+)CERT";
+
+//The mqtt server may require client certificate for authentication.
+// The following are genereted and signed thanks to openssl.
+// The signing certificate is holded by the mqtt server.
+// Please see Client section in https://mosquitto.org/man/mosquitto-tls-7.html
+const char cert_client [] PROGMEM = R"CERT(
+-----BEGIN CERTIFICATE-----
+... ... ... ... ...
+-----END CERTIFICATE-----
+)CERT";
+
+const char key_client [] PROGMEM = R"CERT(
+-----BEGIN RSA PRIVATE KEY-----
+... ... ... ... ...
+-----END RSA PRIVATE KEY-----
+)CERT";
+// end of certificate chain
+////////////////////////////////////////////////////////////