Feature/add validator for path parameters not present in path

[DCollart]

Fixes #523

For that new rule I used the "PathString" of the context in order to do the check.

I could also maybe have put that rule at the "Path" level, what do you think ?

[ghost]

All CLA requirements met.

[darrelmiller]

Doing the validation at the parameter level is better. There is an exception that was introduced in 3.1 that says if the path Item is empty, then the required parameter rule is not applied. https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#path-templating