Also renamed error type MQTT_ERROR_TYPE_ESP_TLS to
MQTT_ERROR_TYPE_TCP_TRANSPORT, as the former was confusing and might
have implied the error occurred in TLS layer.
Applicable IDF version >= v4.3
Closes https://github.com/espressif/esp-idf/issues/5906
Digital Signature enables hardware accelerated RSA signature
for TLS handshake.The RSA private key(client key) is also stored in
encrypted format and ecryption key is stored in hardware(efuse) which adds
additional level of security for mutual authentication.
* Digital Signature is only supported for ESP32-S2.
Applicable IDF version >= v4.3
ii) fix secure_element error return
The feature allows use of secure element for TLS connections, which makes use of hardware security for storage of client private keys(only keys with ECC algorithm)
Applicable IDF versions: >= 4.2
