Implement common diagnostics

[norro]

Populate the micro_ros_common_diagnostics package with common diagnostics / monitors for microcontrollers, e.g., heap monitor, e.g., for ESP32, etc.

Monitors for specific platforms should be possible to enable/disable via cmake flag.

[norro]

@bjv-capra Do you have implementations of updaters/monitors that could fit into this repository and that you are willing to open-source?

[bjv-capra]

None at the moment, unfortunately. The ones we have are too specific at the moment.