ValidationRules should maintain state between validation and message generation

[Ambro17]

Problematic Code:
https://github.com/Ambro17/graphql-schema-diff/blob/master/schemadiff/validation.py#L34-L35

As the object is instantiated two times, any atributes set on the validation step will not be available while building the message.

This causes code repetition between validation and message generation when they need to provide extra details about why that change was restricted. For example: The common prefixes shared between two arguments