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 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 69.06%. Comparing base (641c929) to head (afc34ba).

Files with missing lines	Patch %	Lines
include/control_filters/exponential_filter.hpp	0.00%	28 Missing ⚠️
src/control_filters/exponential_filter.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           ros2-master     #231      +/-   ##
===============================================
- Coverage        71.05%   69.06%   -2.00%     
===============================================
  Files               20       23       +3     
  Lines             1040     1070      +30     
  Branches            84       88       +4     
===============================================
  Hits               739      739              
- Misses             255      285      +30     
  Partials            46       46              

Flag	Coverage Δ
unittests	69.06% <0.00%> (-2.00%)	⬇️

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Files with missing lines	Coverage Δ
src/control_filters/exponential_filter.cpp	0.00% <0.00%> (ø)
include/control_filters/exponential_filter.hpp	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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