Factor out AD-specific code into extensions

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b676248) 92.30% compared to head (3a7aff4) 92.30%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #20 +/- ## ======================================= Coverage 92.30% 92.30% ======================================= Files 5 7 +2 Lines 117 117 ======================================= Hits 108 108 Misses 9 9 ``` | [Flag](https://app.codecov.io/gh/lassepe/ParametricMCPs.jl/pull/20/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Lasse+Peters) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/lassepe/ParametricMCPs.jl/pull/20/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Lasse+Peters) | `92.30% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Lasse+Peters#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.