Add the possibility to add custom field name as string

[JulienRAVIA]

I'm doing this PR because i've noticed that when i want to add a custom field, if there is only entry in the $name (expecting array) parameter, the custom_field won't have his name reported in Payrexx or the webhooks :

Problem :

Does not work :

$gateway->addField($type = 'custom_field_2', $value = '123456789', $name = [
	'Identifier'
]);

I guess it expect to have an array with an index with country codes, but in my case i don't care if the custom field is translated, and i don't want to have to translate it in any language :)

Solution :

But when i do this, it does work :

$gateway->addField($type = 'custom_field_2', $value = '123456789', $name = 'Identifier');

But when i do this, my PHPStan doesn't like this at all :

This PR so resolve it 🙏

As you see, it's not a big deal ;)