[OpenAPI/Hydra] Validation error schema should be generated

**Description**

In `OpenApiFactory`, the schema is not generated for the validation error (422):
https://github.com/api-platform/core/blob/e649fa1507963102fddb339458a82c426240553b/src/OpenApi/Factory/OpenApiFactory.php#L227

It would be nice to add it at least for Hydra.
Hydra normalizes the validation error like this:
https://github.com/api-platform/core/blob/e649fa1507963102fddb339458a82c426240553b/src/Core/Hydra/Serializer/ConstraintViolationListNormalizer.php#L46-L50

One way to implement it would be to add a new schema type, like `TYPE_VALIDATION_ERROR` here:
https://github.com/api-platform/core/blob/e649fa1507963102fddb339458a82c426240553b/src/Core/JsonSchema/Schema.php#L30-L31

And handle this new type in this method:
https://github.com/api-platform/core/blob/e649fa1507963102fddb339458a82c426240553b/src/Core/Hydra/JsonSchema/SchemaFactory.php#L73