Add filter plugin for exponential filter

[christophfroehlich]

Similar to control_filters/LowPassFilter, the filter itself was already there.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 47.36842% with 10 lines in your changes missing coverage. Please review.

Project coverage is 51.06%. Comparing base (7dda65f) to head (d877ba6).

Files with missing lines	Patch %	Lines
include/control_filters/exponential_filter.hpp	47.36%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           ros2-master     #231      +/-   ##
===============================================
- Coverage        51.20%   51.06%   -0.15%     
===============================================
  Files               10       11       +1     
  Lines              496      515      +19     
  Branches            59       60       +1     
===============================================
+ Hits               254      263       +9     
- Misses             215      224       +9     
- Partials            27       28       +1     

Flag	Coverage Δ
unittests	51.06% <47.36%> (-0.15%)	⬇️

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Files with missing lines	Coverage Δ
include/control_filters/exponential_filter.hpp	47.36% <47.36%> (ø)