New package dynamicvalidator

[magodo]

This PR adds a new package dynamicvalidator to support cross attribute validations.

[austinvalle]

Thanks for getting this package bootstrapped @magodo 🙌🏻 , just a couple suggestions to fix the compilable examples.

We also will need a changelog if you'd like to generate that and push it to your branch.

[magodo]

@austinvalle Thank you! I've updated the code accordingly, please take another look!

[austinvalle]

Thanks again @magodo! I'm going to push a changelog to your branch + remove one of the comments the example, then will merge + release this.