Added client support for self signed certificate on MQTT server

[Sjd-Risca]

I'd like to use the mqtt client against a server with a self signed Certificate Authority. The proposed patch will add a new client configuration attribute defined as ca_certs for assigning a custom CA (instead of using the system ones) to the ssl context. The connection will be initiated only if the mqtt server's certificate chain resolved to any of the custom CAs.

For reference, see the implementation from patho-mqtt: https://github.com/eclipse/paho.mqtt.python/blob/225ab3757f6818ba85eb80564948d1c787190cba/src/paho/mqtt/client.py#L767

[HerrMuellerluedenscheid]

Nice! Can you think of a unit test for this feature? Maybe creating a tempfile and a certificate and loading that? I mean your changes use external logic, thus nothing should go wrong here. But it comes very handy when adding functionality.

[FlorianLudwig]

Hi @Sjd-Risca

hbmqtt has been deprecated by it's author. We created a fork here: https://github.com/Yakifo/amqtt

I like this change and would like to include it in our fork.

Would you be interested to create your MR there as well?

Thanks :)

[FlorianLudwig]

@HerrMuellerluedenscheid would you be interested in carrying this forward?