Introduce FcmTopicChannel

[dwightwatson]

This introduces a new FcmTopicChannel (see #192) that allows sending FCM notifications to topics. It's important to understand why this is implemented as a separate channel and is designed to work a little differently to other notifications that are sent to notifiable models.

My general expectation is that this would be used as an on-demand notification where you would dispatch the notification to a topic in real-time.

use NotificationChannels\Fcm\FcmTopicChannel;

Notification::route(FcmTopicChannel::class, 'news')
    ->notify(new BlogCreated($blog));

However - you can still implement routeNotifiationForFcmTopic on a model and send a notification that way, or set the topic on the message itself. Perhaps this could be expanded to support multiple topics at the same time (if the API allows) but I chose not to implement this here.

I updated the docs, cleaned up some additional code and also adopted Pint for consistency going forward.