F fix two doc validation

Summary

Fixes the missing validation of custom converters defined in the separate metadata.

Focus

Mainly just adds the Converters section from the metadata document to the schema. Also refactors Crawler.load_definition a little for better unit testing.

Test Environment

New unit test should be sufficient but feel free to try with any other two-document cfood definition.

Check List for the Author

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• All automated tests pass
• Reference related issues
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  • Problems with old code
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Check List for the Reviewer

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• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
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