Switch to pydantic

[liiight]

Finally switch the schema engine to pydantic.

To do:

• Refactor all providers
• Handle the validation errors better
• Make sure to support issue Boolean values taken from environment variables raise BadArguments #387
• Update the tests
• Update Changelog
• Refactor CLI

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (2.0.0@5a45352). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             2.0.0     #359   +/-   ##
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  Coverage         ?   84.57%           
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  Files            ?       56           
  Lines            ?     2749           
  Branches         ?        0           
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  Hits             ?     2325           
  Misses           ?      424           
  Partials         ?        0           

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