Added support for inheritance hierarchies validation

[ArielBerkovich]

added constraintOnClass api to resolve issue: #278

this provides convenient API to create a parent class validator that can also validate different inherited classes extending it.

example:

given a class called Child that extends Parent, one could utilize constraintOnClass in the following way:

    ValidatorBuilder.<Parent>of()
                // constraints for parent
                .constraint(...)
                .constraintOnClass(Child.class), b -> b
                            // constraints for child
                            .constraint(...)
                            .constraint(...)
                            .constraint(...)
                    )
                )

[making]

target name cannot be determined automatically.

[ArielBerkovich]

I'm a bit puzzled, actually, since I cant think of a target name that makes sense other than the class name.
that is because unlike regular .nest() usage, I am not validating a nested field, but rather the whole child class.

perhaps using nest() is some sort of an abuse here, given that I use it as work around to cast my entity, not access a nested field.

I'll see if I can come up with a different approach.

[ArielBerkovich]

ended up creating a new Validatable class dedicated to this case called InheritanceValidator, would love to hear your thoughts

[making]

I meant that the target name should be a parameter.
I don't think InheritanceValidator is needed.

[ArielBerkovich]

Understood, I appreciate your review by the way. I find Yavi's design and philosophy thoroughly engaging.

I agree that the target name could have been a parameter, my concern arises from utilizing nest() for non-nested fields, and determining the appropriate target name for a child class.

what do you think?

[making]

The test case is difficult to understand.

Please add the simple example in the pull request to the test case.

[ArielBerkovich]

sure, no problem.

[making]

These methods are quite advanced. Can you please add to the JavaDoc what use cases it is useful for and some sample code?

[ArielBerkovich]

absolutely

[making]

It is not enough to simply verify the results of validation. Please also verify the contents of the constraints (name and messageKey).