Add joint names as std::string to mimic struct

[christophfroehlich]

In the handles and API for the hardware components we use vector of joint names and also unordered maps with joint names keys.
While I was working on #2571 I found it helpful to have also the string names in the mimic struct, instead of the indices only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (56fc052) to head (b2f77cc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2575   +/-   ##
=======================================
  Coverage   89.31%   89.31%           
=======================================
  Files         145      145           
  Lines       16541    16543    +2     
  Branches     1396     1396           
=======================================
+ Hits        14773    14775    +2     
  Misses       1231     1231           
  Partials      537      537           

Flag	Coverage Δ
unittests	89.31% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...rface/include/hardware_interface/hardware_info.hpp	81.48% <ø> (ø)
hardware_interface/src/component_parser.cpp	93.91% <100.00%> (+0.02%)	⬆️
hardware_interface/test/test_component_parser.cpp	98.98% <ø> (ø)

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