JSON Schema / Spec / Docs for the Malware filter list data provider API

There should be a specification publicly available for the format we support for ingesting package malware reports, as currently implemented in https://github.com/composer/packagist/blob/main/src/FilterList/List/AikidoMalwareFilterList.php.

Start by reading src/FilterList/List/AikidoMalwareFilterList.php to identify the currently supported package malware report format. Document that format as a public specification, including a JSON Schema if appropriate, and ensure the resulting documentation matches the implementation.

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