Add optional "groups" attribute to @Valid

At the moment, when using @Valid annotation, there is no way to specify which groups must be validated. Inspired by the [Springs @Validated annotation](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/validation/annotation/Validated.html), my suggestion is to add an optional attribute to @Valid annotation, "groups", where we can define which groups should be validated.

Start with the @Valid annotation definition in the validation specification and compare its semantics with the Spring @Validated documentation linked in the issue. Determine how an optional groups attribute would fit the existing validation-group behavior, then clarify the intended design and update the specification once the compatibility criteria are agreed.

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