fix: json validation allow arbitrary $schema value

[jkowalleck]

fixes #612

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ +0.00%	✅ ∅ (target: 80.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (0d00496)	4472	4153	92.87%
Head commit (ddc8181)	4472 (+0)	4153 (+0)	92.87% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#613)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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