Add notifier assertions trait

[d-mitrofanov-v]

Added assertions for notifier based on symfony NotificationAssertionsTrait

[TavoNiievez]

@d-mitrofanov-v Thank you for taking the time to add these new assertions!

All of these assertions were included in Symfony 6.2, and are not available in Symfony 5.4.

This module maintains the same support as the supported versions of Symfony, so we must ensure compatibility with Symfony 5.4 while it is still supported.

For a long time, I didn't think creating compatibility code was the most elegant option for this, which is why I kept putting off adding these assertions, but waiting until 2029 for SF 5.4 to become unsupported seems worse tbh.

So, Please add conditionals to each assertion to check that if the current version of Symfony is 5.4, you throw an exception explaining that these assertions are only available from SF 6.2 onwards (although in reality we only support 6.4+).

[TavoNiievez]

You can use Symfony\Component\HttpKernel\Kernel::VERSION_ID to make this comparison, or use version_compare with Kernel::VERSION, both options are fine.

[d-mitrofanov-v]

@TavoNiievez Thanks for review!

I've added check for a symfony version in getNotificationEvents() method as all assertions use it directly or indirectly

[TavoNiievez]

LGTM!