Move JsonSchema behind a Cargo feature

[adamchalmers]

Deriving JsonSchema adds compilation time, and not every consumer of this library needs it.

Do not merge until I've got integration PRs open for this in kcl-lib.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1446   +/-   ##
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  Coverage        ?   36.12%           
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  Files           ?       32           
  Lines           ?     1733           
  Branches        ?        0           
=======================================
  Hits            ?      626           
  Misses          ?     1107           
  Partials        ?        0           

Flag	Coverage Δ
unittests	36.12% <ø> (?)

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