Launch descriptions for system modes

[norro]

E.g.

• change system mode of a node ✓
• change state and system mode of a system ✓
• event handler for performed mode changes of nodes ✓
• event handler for performed state changes and mode changes of systems ✓

[codecov[bot]]

Codecov Report

Merging #80 (e3dae5e) into master (241fcb4) will increase coverage by 1.24%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   31.29%   32.53%   +1.24%     
==========================================
  Files          14       14              
  Lines        1048     1048              
  Branches      754      754              
==========================================
+ Hits          328      341      +13     
+ Misses        147      118      -29     
- Partials      573      589      +16     

Impacted Files	Coverage Δ
system_modes/src/system_modes/mode_observer.cpp	20.00% <0.00%> (ø)
system_modes/src/system_modes/mode_manager.cpp	18.42% <0.00%> (+1.87%)	:arrow_up:
system_modes/src/system_modes/mode_inference.cpp	37.05% <0.00%> (+2.17%)	:arrow_up:

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[norro]

This PR introduces two new related packages: launch_system_modes and test_launch_system_modes. See launch_system_modes/README for the introduces features.

The general idea is to allow working with system modes in ROS 2 launch files as shown in the newly introduces example launch files (example_system_start_drive_base.launch.py and example_system_started.launch.py) and tested within the test_launch_system_modes package.

Provides features/fix for https://github.com/micro-ROS/system_modes/issues/79