Add Notification Channel Extender

[SychO9]

Fixes #2419

Changes proposed in this pull request:

• Use drivers for the different channels, each driver takes care of enqueing the notification job and adding the user preference.
• Add an extender for adding new drivers.
• Deprecate Flarum\Notification\Event\Sending Event.
• Add some dockblocks for the notification extender.

Reviewers should focus on:

• Are the drivers being properly registered.
• Class/Interface/Method naming.

Confirmed

• Backend changes: tests are green (run composer test).

Required changes:

• Related core extension PRs: Use a notification driver with the new channel extender pusher#28

[askvortsov1]

One interesting point brought up by flarum/pusher#28 is that each driver will end up queing its own, independent jobs. Do we want to split it up like this, or move the logic for running through the drivers to inside the event?

[clarkwinkelmann]

Looking good code-wise.

I see the tests only test against the container bindings, not for the actual sending of notifications.

I'd love to see this tested with an actual use case, like the Telegram notifications of the Telegram login extension before being shipped, so we make sure we ship something that will be practical in the real world. Or was a test already performed on another real world extension?

[askvortsov1]

Do you mean in the integration tests? If you mean manual testing, I'm planning to test this with my PWA extension before merging

[SychO9]

there's technically also the pusher extension which I tested on (if that counts).

[askvortsov1]

I can confirm that this works with my PWA extension, for both preferences and actually sending notifications.