docs: add changelog for PR 12581 (JSON schema)

changes/ee/feat-12581.en.md

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+Add JSON schema to schema registry.
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+JSON Schema supports [Draft 03](http://tools.ietf.org/html/draft-zyp-json-schema-03), [Draft 04](http://tools.ietf.org/html/draft-zyp-json-schema-04) and [Draft 06](https://datatracker.ietf.org/doc/html/draft-wright-json-schema-00).