[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 62.10526% with 36 lines in your changes missing coverage. Please review.

Project coverage is 87.42%. Comparing base (d55def1) to head (efd8cbf).

Files with missing lines	Patch %	Lines
...de/hardware_interface/loaned_command_interface.hpp	48.57%	15 Missing and 3 partials ⚠️
...lude/hardware_interface/loaned_state_interface.hpp	41.17%	8 Missing and 2 partials ⚠️
...re_interface/include/hardware_interface/handle.hpp	81.39%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1750      +/-   ##
==========================================
- Coverage   87.62%   87.42%   -0.20%     
==========================================
  Files         120      120              
  Lines       12217    12310      +93     
  Branches     1093     1105      +12     
==========================================
+ Hits        10705    10762      +57     
- Misses       1123     1150      +27     
- Partials      389      398       +9     

Flag	Coverage Δ
unittests	87.42% <62.10%> (-0.20%)	⬇️

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

Files with missing lines	Coverage Δ
...re_interface/include/hardware_interface/handle.hpp	88.73% <81.39%> (-11.27%)	⬇️
...lude/hardware_interface/loaned_state_interface.hpp	57.57% <41.17%> (-17.43%)	⬇️
...de/hardware_interface/loaned_command_interface.hpp	58.13% <48.57%> (-41.87%)	⬇️

... and 3 files with indirect coverage changes

[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?

[destogl]

@mamueluth please check this. This is how we can template the access to variables. This should simplify test changes.