refactor: fetch extensions

[gadomski]

@ceholden this is a bit of a simpler implementation. I wasn't finding a fetch_extensions setup that felt right to me. I tested locally by deleting my extensions and running just the no_install test and it failed 🥳

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.02%. Comparing base (973414b) to head (7c9d5ca).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #681      +/-   ##
==========================================
+ Coverage   41.98%   42.02%   +0.03%     
==========================================
  Files          47       47              
  Lines        2751     2751              
==========================================
+ Hits         1155     1156       +1     
+ Misses       1596     1595       -1     

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[ceholden]

+1 to the design in general, especially as the DuckDB config has all of these in the same place

[ceholden]

Thanks! I think this looks great