move DSP to an extension

[baggepinnen]

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[JuliaControlBot]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.78%. Comparing base (a7c7e6a) to head (9e7a12f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/ControlSystemsBase.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #960      +/-   ##
==========================================
- Coverage   92.80%   92.78%   -0.02%     
==========================================
  Files          41       41              
  Lines        4974     4975       +1     
==========================================
  Hits         4616     4616              
- Misses        358      359       +1     

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