Re-introduce extensions for AD backends

[lassepe]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.51%. Comparing base (57ce8d5) to head (4014535).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/InternalAutoDiffUtils.jl	80.95%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   86.51%   86.51%           
=======================================
  Files           7        9    +2     
  Lines         178      178           
=======================================
  Hits          154      154           
  Misses         24       24           

Flag	Coverage Δ
unittests	86.51% <90.00%> (ø)

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