Validators contravariance

[javier-godoy]

ValidatorSupport.getValidators() should return a List<Validator<? super T>> instead of List<Validator<T>>, because PECS.

https://github.com/AppJars/commons-backend/blob/ca5a44fea72cec4a41f3b5c240bd8da590a43b0c/commons-model/src/main/java/com/appjars/saturn/validation/ValidationSupport.java#L27

_{(Picture courtesy: Andrey Tyukin CC-BY-SA)}

For instance, if MyEntity implements HasFoo and FooValidator implements Validator<HasFoo>, then MyEntityService can implement getValidators() as:

	public List<Validator<? super MyEntity>> getValidators() {
		return List.of(new FooValidator());
	}

(which makes sense because FooValidator is able to validate any Foo, including my entity)