feat!: use a different JSON schema validator

[jkowalleck]

currently using opis/json-schema - https://packagist.org/packages/opis/json-schema

let's switch to a more popular JSON schema validator: justinrainbow/json-schema - https://packagist.org/packages/justinrainbow/json-schema

this is considered a breaking change, as such a switch might introduce unintended issues with schema validation.