feat(mqtt): Implement MQTT TLS encryption using certificates

[Slider0007]

• Implement TLS encryption for MQTT connection using certificates
  • Three certificate files are necessary on client side:
    • Certificate Authority (CA) certificate, e.g. ca.crt (unencyrpted, no password)
    • Client certificate, e.g. client.crt (unencrypted, no password)
    • Client provate key, client.key (unencrypted, no password)
  • Common certificate file extentions: *.crt, *.pem, *.der
  • Certificate files shall be copied to SD card folder /config/certs
  • 'certs' subfolder is generated automatically by firmware if missing
  • Certificate 'common name' (CN) check is disabled by default (hard-coded)
    mqtt_cfg.broker.verification.skip_cert_common_name_check = true
• Add some additional error handling to indicate possible setup issues
• Heap allocation in external SPIRAM to limit internal RAM usage
• New parameter added in MQTT section (expert level)
  

NOTE: A server/broker certificate implementation is mandatory to operate ESP with TLS encryption, refer to the MQTT broker manual and e.g. for certificate creation https://mosquitto.org/man/mosquitto-tls-7.html)

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Feature ported from jomjol#2651