Hide component parser api

[Karsten1987]

as per title.

the public API of the component parser currently introduces details, which are not necessary. Essentially hides tinyxml2.
The diff looks more funky than it actually is. The .cpp file got a namespace detail and all tinyxml related functions are part of it. No code got modified.

[codecov-commenter]

Codecov Report

Merging #157 into master will not change coverage.
The diff coverage is 40.00%.

@@           Coverage Diff           @@
##           master     #157   +/-   ##
=======================================
  Coverage   36.61%   36.61%           
=======================================
  Files          42       42           
  Lines        1726     1726           
  Branches     1080     1080           
=======================================
  Hits          632      632           
  Misses        100      100           
  Partials      994      994           

Flag	Coverage Δ
#unittests	36.61% <40.00%> (ø)

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Impacted Files	Coverage Δ
hardware_interface/src/component_parser.cpp	41.30% <40.00%> (ø)

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[bmagyar]

Thumbs up for reducing exported interfaces