[12.x] Add enum support to notification locale method

[amirhshokri]

This PR enhances the locale() method by allowing it to accept enum values in addition to strings.

Usage example:

//Before
$user->notify((new InvoicePaid($invoice))->locale(LOCALE::EN->value));

//After
$user->notify((new InvoicePaid($invoice))->locale(LOCALE::EN));