Gate fuzz oracle targets behind features

[LucaCappelletti94]

The fuzz crate compiled bundled SQLite, pg_query and DuckDB for every target, so building fuzz_parse_sql alone took 175 s from a cold target directory. Each engine now sits behind an optional feature (sqlite, postgres, duckdb) that only its oracle target requires, which brings the same build to 70 s. ClusterFuzzLite builds with --all-features and still ships all five targets, checked locally through the OSS-Fuzz compile entrypoint.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.02%. Comparing base (9352883) to head (628f185).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2585   +/-   ##
=======================================
  Coverage   81.02%   81.02%           
=======================================
  Files          42       42           
  Lines       33455    33455           
  Branches    33455    33455           
=======================================
  Hits        27108    27108           
  Misses       2789     2789           
  Partials     3558     3558           

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[EthanVieira]

lgtm!