[Enhancement] ValidationBehavior should support async mode.

[AndreiMisiukevich]

Summary

API Changes

I propose to replace protected method bool Validate(object value) by Task ValidateAsync(object value)

Intended Use Case

Allow to create custom validators that work in async mode (e.g. send requests to the server)

Who Will Do The Work?

• I am willing to take this on myself
• Just putting this out there for someone to pick up

[AndreiMisiukevich]

Implemented in #912