v3.0.0: Remove validator overwrite (#63)

* Extract validation logic into own class

* Deprecate the custom validator

The is no need for a custom validator instance. It is bad practice for 3rd party packages to define rules this way.

* Adapt service provider, install validation rule as "extension"

Also, removed the custom resolver  - the logic that is overwriting an application's  validator instance.

* Add test helper

Helper is responsible for creating validator instance with desired validation rules...

* Adapt unit tests

* Remove IDE injected PHPDoc

* Fix style