Add support for subtitle field (iOS)

[hramos]

Expo push notifications can take a subtitle field that is passed on to iOS devices. This PR adds support for this field.

Test Plan

Verify in our own app

We're using laravel-notification-channels/expo in our application. I copied the changes to ExpoMessage.php from this PR into the vendor directory, and updated our code to set the subtitle field:

        return ExpoMessage::create()
            ->title($title)
            ->subtitle($subtitle)
            ->body($body)
            ->enableSound()
            ->setJsonData($jsonData)
            ->badge(1);

I was able to confirm that the push notification that is sent via Expo does in fact include a subtitle ("Notification Tests" in this case):

Run tests

$ composer test
> phpunit
PHPUnit 8.5.33 by Sebastian Bergmann and contributors.

................                                                  16 / 16 (100%)

Time: 78 ms, Memory: 14.00 MB

OK (16 tests, 17 assertions)

[nicko170]

Thanks @hramos - Pushed in 1.3.0.