Add very basic example for mqtt tls

[mrkeuz]

Tested on next setup:

• MicroPython 1.17 (without ca-cert's)
• Traefik Proxy 2.5 (Let's Encrypt Cert)
• Mosquitto 2.0.12

Note:

• Unable connect without ssl_params = {"server_hostname": "<addr>"}

• If keepalive = 0 (default) got MQTTException(2)

• Don't found ssl.CERT_XXX or ussl.CERT_XXX constants, although the constants are specified in the documentation. Only found in inner definitions. But didn't understand how use it outside: https://github.com/micropython/micropython-lib/blob/3c383f6d2864a4b39bbe4ceb2ae8f29b519c9afe/python-stdlib/ssl/ssl.py#L5-L7

  Not sure if all of this is bad defaults or this behavior made by design.