examples/mqttc: add option to use MBED TLS for connections

[pkarashchenko]

Summary

Add mbedTLS option to examples/mqttc.
I haven't tried to connect with this code exactly to "broker.hivemq.com", so example may not work as is (somebody needs to try), but I used this code to connect to other MQTT server with TLS.

This code has some advantages compared to native MQTT-C mbedtls_sockets.h implementation in the way that it tolerates self-signed certificates on a server (usually useful during development).

Impact

Extend MQTT-C example

Testing

Tested on ESP32-S3, but connecting to custom MQTT provider service

[nuttxpr]

[Experimental Bot, please feedback here]

The PR summary is missing some key information:

• Why is this change necessary? Just saying it "adds an option" isn't enough. Explain the benefits of using mbedTLS (e.g., enhanced security) and why this is a good addition to the example.
• What functional part of the code is being changed? Be more specific than "Extend MQTT-C example." Which files are modified? Are new files added?
• How does the change exactly work? Briefly describe how the mbedTLS option is integrated and used within the example.

Impact:

• Impact on user: The PR description claims the example "may not work" with the provided broker. This needs clarification. Will users need to modify the code to connect to different brokers? If so, how? This needs to be clearly documented.
• Impact on build: Does adding the mbedTLS option require any changes to the build system or configuration? This should be stated explicitly.
• Impact on documentation: The description mentions that the code tolerates self-signed certificates, which is important for users. This, along with any other necessary usage instructions, needs to be added to the documentation.

Testing:

• Insufficient information: The PR only mentions testing on ESP32-S3 with a custom MQTT provider. To be considered well-tested, it needs to be validated on other supported platforms and ideally with the provided "broker.hivemq.com" example.
• Missing logs: The "Testing logs before change" and "Testing logs after change" sections are empty. Provide actual logs demonstrating the functionality before and after your changes.

In conclusion, this PR does not yet meet the NuttX requirements. You need to address the missing information and expand on the testing before it can be considered for merging.

[cederom]

Thank you @pkarashchenko :-)

• Can you please update also the documentation part so everyone knows that MQTT + TLS is supported?
• What happens when self-signed certificate is in use? How can we control reaction to that situation? Shall we create a dedicated CONFIG variable to accept self-signed certificates during development? In general self-signed certificates on a production system indicates compromised channel so we should not allow that by default :-)

[cederom]

Thank you @pkarashchenko :-)

Ci check is a GH worker problem.

Please consider adding documentation in a free moment, especially the self-signed certificates handling / warning / errors :-)

[pkarashchenko]

Let me update the PR

[xiaoxiang781216]

@pkarashchenko please rebase to the latest master to fix check error.

[pkarashchenko]

@cederom I added CONFIG_EXAMPLES_MQTTC_ALLOW_UNVERIFIED_TLS option.

[pkarashchenko]

Let's ignore error: Mixed case identifier found

[acassis]

@pkarashchenko please include a board profile, for example sim:mqtt_tls

[pkarashchenko]

@pkarashchenko please include a board profile, for example sim:mqtt_tls

Will do. As well as a docs update