Implement EZP-21095: LoadUser by login and email 

This pull request enhances and closes #416.

PR adds the `loadUserByLogin` & `loadUsersByEmail` method to the `UserService`.

## Usage:
```php
$repository    = $this->getRepository();
$userService = $repository->getUserService();

$userService->loadUserByLogin( 'admin' ); 
    #=>  \eZ\Publish\API\Repository\Values\User\User


$userService->loadUsersByEmail( 'nospam@ez.no' ); 
    #=>  \eZ\Publish\API\Repository\Values\User\User[]
```

## TODO
- [x] Change SPI to split loading login from email
- [x] Add API to loadByLogin
- [x] Add API to loadByMail
- [x] Add (and fix?) tests