Add new get_value API for Handles and Interfaces

[saikishor]

Related to ros-controls/ros2_controllers#1442 and ros-controls/ros2_controllers#1443

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.59%. Comparing base (4ad5709) to head (4403195).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...re_interface/include/hardware_interface/handle.hpp	50.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1976      +/-   ##
==========================================
- Coverage   88.61%   88.59%   -0.03%     
==========================================
  Files         122      122              
  Lines       13304    13314      +10     
  Branches     1207     1209       +2     
==========================================
+ Hits        11789    11795       +6     
- Misses       1062     1064       +2     
- Partials      453      455       +2     

Flag	Coverage Δ
unittests	88.59% <60.00%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
...de/hardware_interface/loaned_command_interface.hpp	52.63% <100.00%> (+1.28%)	⬆️
...lude/hardware_interface/loaned_state_interface.hpp	58.82% <100.00%> (+1.24%)	⬆️
hardware_interface/test/test_handle.cpp	100.00% <100.00%> (ø)
...re_interface/include/hardware_interface/handle.hpp	83.75% <50.00%> (-4.99%)	⬇️

... and 3 files with indirect coverage changes