Add pUserData field to MQTTContext_t for application use

[guclumhg]

Description

Adds an application-defined void * pUserData field to MQTTContext_t, as requested in #256. The library never reads or modifies the field; it lets the application (for example a C++ wrapper passing its this pointer) access its own state from the MQTTEventCallback_t callback through the pContext parameter. Since MQTT_Init clears the whole context, the field is documented to be set after calling MQTT_Init.

A unit test verifies that MQTT_Init clears the field and that the value set by the application is visible inside the event callback when a QoS 1 PUBLISH is received.

There is no behavioral change for applications that do not use the field, and coverage is unchanged (97.5% lines / 93.8% branches).

Happy to rename the field or switch to a setter/getter API if the maintainers prefer.

Test Steps

Both CI unit-test configurations pass locally (5/5 test suites), including the new test_MQTT_Init_UserData_AccessibleInEventCallback:

cmake -S test -B build -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Debug \
  -DBUILD_CLONE_SUBMODULES=ON -DUNITTEST=1 [-DCOV_ANALYSIS=1] \
  -DCMAKE_C_FLAGS='--coverage -Wall -Wextra -Wsign-compare -Werror -DLIBRARY_LOG_LEVEL=LOG_DEBUG'
make -C build all
ctest --test-dir build -E system --output-on-failure

Formatting verified with uncrustify 0.69.0 using the config from FreeRTOS/CI-CD-Github-Actions.

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

Fixes #256

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