[DX] Ability to authentication a User directly

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11158
| License       | MIT

Simple usage example:

```php
class LoginController extends ContainerAware
{
    public function loginAction()
    {
        $user = new User('norzechowicz', 'password123');
        $this->container->get('security.login_manager')->loginUser('firewall_name', $user);

        return new Response();
    }
}
```
For `FakeRememberMeServicesResolver`, was there a reason you didn't just mock it in the test?

And if you'd like to replace the `RememberMeServicesResolver`, you could still sub-class it obviously. It's only used with this new LoginManager, which is meant just as a helper service anyways.

I can see it either way, but I don't think we really lose anything with removing the interface.