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 72.75%. Comparing base (e1f26e4) to head (3b036df).

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        74.78%   72.75%   -2.04%     
===============================================
  Files               22       25       +3     
  Lines             1071     1101      +30     
  Branches            85       89       +4     
===============================================
  Hits               801      801              
- Misses             222      252      +30     
  Partials            48       48              

Flag	Coverage Δ
unittests	72.75% <0.00%> (-2.04%)	⬇️

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

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

[saikishor]

In general looks great.
Thanks for this new filter