Uses constants instead hardcoded strings for payment types names in configs

The one of best practices is to use constants for static names. It helps to avoid misspelling and simplifies refactoring later. Example of usage:

```php
<?php
class MyConfigProvider implements ConfigurationProvider
{
  //..
  public function getUsername($type, $country) {
    if ($type == ConfigurationProvider::INVOICE_TYPE) {
      return 'my-invoice-username';
    }
  }
}

```