Add Async Function Handler

[saikishor]

The async function handler is moved from ros-controls/ros2_control#1489

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.40580% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (46b4c44) to head (305f3b8).

Files	Patch %	Lines
include/realtime_tools/async_function_handler.hpp	88.40%	3 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   76.37%   80.39%   +4.02%     
==========================================
  Files           7        8       +1     
  Lines         237      306      +69     
  Branches       40       53      +13     
==========================================
+ Hits          181      246      +65     
+ Misses         26       25       -1     
- Partials       30       35       +5     

Flag	Coverage Δ
unittests	80.39% <88.40%> (+4.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
include/realtime_tools/async_function_handler.hpp	88.40% <88.40%> (ø)

... and 1 file with indirect coverage changes