[HW_IF] Prepare the handles for async operations

[saikishor]

As with #1688 and #1689 there are not many changes in the handles.hpp file. I think this is the right moment to do it, to have straight away working handles in async. The proposed changes would work with zero overhead in synchronous operations.

Looking forward for your feedback!

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 14 lines in your changes missing coverage. Please review.

Project coverage is 87.12%. Comparing base (ce5c35a) to head (facb6a7).

Files with missing lines	Patch %	Lines
...re_interface/include/hardware_interface/handle.hpp	80.48%	4 Missing and 4 partials ⚠️
...de/hardware_interface/loaned_command_interface.hpp	42.85%	2 Missing and 2 partials ⚠️
...lude/hardware_interface/loaned_state_interface.hpp	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1750      +/-   ##
==========================================
- Coverage   87.19%   87.12%   -0.08%     
==========================================
  Files         118      118              
  Lines       11369    11414      +45     
  Branches     1049     1056       +7     
==========================================
+ Hits         9913     9944      +31     
- Misses       1094     1101       +7     
- Partials      362      369       +7     

Flag	Coverage Δ
unittests	87.12% <73.07%> (-0.08%)	⬇️

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

Files with missing lines	Coverage Δ
...lude/hardware_interface/loaned_state_interface.hpp	60.00% <50.00%> (-6.67%)	⬇️
...de/hardware_interface/loaned_command_interface.hpp	71.42% <42.85%> (-28.58%)	⬇️
...re_interface/include/hardware_interface/handle.hpp	88.40% <80.48%> (-11.60%)	⬇️

[saikishor]

I've just gone through the coverage report, the missing coverage lines should already be handled in the testing of #1489 and #1567

[mergify]

This pull request is in conflict. Could you fix it @saikishor?