Adding user configuration file for mqtt5 options

[RLeclair]

User Configuration File for MQTT5 options:

• Merged all the MQTT5 options and connection options into a single file called az_mqtt5_config.h
• Users can now specify a configuration file directory through CMake, and the config file must be named az_mqtt5_user_config.h.

Rationale:

• Instead of having configuration values scattered across MQTT5 and MQTT5 connection options, consolidating them in one file reduces code bloat.
• Many options only need to be set once, and those that change are still kept in the options (e.g., port number for az mqtt5).
• A single user configuration file simplifies customization for customers.

Created clone target of az_core

To address unit test requirements, I’ve created a clone target of az_core called az_core_test_lib. This clone uses a separate configuration file. Other tests now depend on az_core_test_lib, and code coverage targets this new clone.

_Note: As the az_mqtt5 files move to a different folder, changes made to azcore might be reverted due to the configuration file’s new location.

[vaavva]

Please add some documentation/example to make adding this configuration file a discoverable process for customers.