Updating the coreMQTT submodule pointer

[DakshitBabbar]

Description

• Updates the coreMQTT submodule pointer to the latest commit of the library.
• Includes the core_mqtt_config_defaults.h file in the core_mqtt_agent_config_defaults.h.
  • This is done because the older version of coreMQTT (v1.x.x) has core_mqtt_config.h included in the core_mqtt.h file whose macros is used by a couple of more files like freertos_command_pool.c in some old demos (like the old OTA demos). But in the new version this includes is removed from the core_mqtt.h file and added in the core_mqtt_config_defaults.h file. A simple solution to this problem was to add the core_mqtt_config_defaults.h in the core_mqtt_agent_config_defaults.h which is included in the core_mqtt_agent.h that is used by the files that require the macros.
• Updates README for correct commands to build unit tests
• Modifies the cmake files for building the unit tests successfully on mac machines
• Modifies the unit tests for full coverage (initially coverage was different for mac and linux)

Test Steps

• Verified the updates with the demos in the core-mqtt-agent-demos repository.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

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