Further Struct based Method changes to support propagation in gz_ros2_control

[soham2560]

Brief

Post #2323 we need to update the downstream repos to also support the new Struct based approach. A PR for gz_ros2_control has been put in at ros-controls/gz_ros2_control#613

Small changes for that to work are present in this PR

[codecov]

Codecov Report

Attention: Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.01%. Comparing base (8c52010) to head (6694e4a).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
hardware_interface/src/resource_manager.cpp	33.33%	10 Missing ⚠️
controller_manager/src/controller_manager.cpp	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2340      +/-   ##
==========================================
+ Coverage   88.98%   89.01%   +0.03%     
==========================================
  Files         147      147              
  Lines       16708    16712       +4     
  Branches     1433     1433              
==========================================
+ Hits        14867    14877      +10     
+ Misses       1285     1279       -6     
  Partials      556      556              

Flag	Coverage Δ
unittests	89.01% <47.61%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
controller_manager/src/controller_manager.cpp	74.33% <83.33%> (+0.22%)	⬆️
hardware_interface/src/resource_manager.cpp	78.28% <33.33%> (ø)

... and 1 file with indirect coverage changes

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