Extend the default ConfigValidator.cs

### What should we add or change to make your life better?

The current `ConfigValidator` is an internal class, which means we cannot use it in our code.
When we try to implement our own validation logic, we have to replace the `IConfigValidator` with our own implementation.

What we want is:
1. Utilize the default validator to perform a set of default validation.
2. Add our extra validation code.

If the `ConfigValidator` is a public class, we can wrap it inside our own validator implementation.

### Why is this important to you?