Add context-aware diagnostic tasks

[Daniiiil1]

Summary

• add rclc_diagnostic_task_init_with_context for callbacks that need instance-specific state
• keep the existing rclc_diagnostic_task_init source-compatible and explicitly initialize its new fields
• dispatch legacy and context-aware callbacks through the same updater path
• replace global state in the website-checker example with a task context
• document the API and add unit coverage for legacy initialization, context delivery, callback execution, and invalid arguments

The API follows the existing rclc pattern: the context pointer is supplied after the callback during initialization and is passed as the callback's final argument. A null context is allowed; a null callback is rejected.

Closes #15.

Validation

• git diff --check
• cppcheck on the changed C/C++ implementation, example, and tests (only pre-existing warnings outside the new code)
• DCO sign-off included

The local machine does not have a ROS 2 / colcon workspace, so the Humble and Rolling builds and gtests are delegated to the repository CI.