[Validator] Add an integer constraint and validator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature? |  yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none that I'm aware of
| License       | MIT
| Doc PR        | will be added when this PR gets approved

Who would think it's so hard to write a validator for an integer that works with both `int` and `string` types like `1` and `'1'`. That's why I decided to add this validator to the core.
@jakzal, okay. But could you first tell me why?