[5.6] [WIP] Add notification database channel custom types

[antonkomarev]

Related to: #23236

This PR introduces the same behavior to the database notification channel to keep notification channels data consistency.

Use case: I'm adding notification to database and broadcasting them to the user. But if user has missed notifications - he can call GET /notifications later and get the same data which was broadcasted earlier. Otherwise notification type will be mismatch and there will be issues with data merge.

The only thing I don't like in this implementation is method naming. Type resolver checks if broadcastType() method exists in Notification. Earlier I suggested name broadcastAs() inspired from Broadcasting Event. Taylor's name broadcastType() is way better, but still coupled with broadcasting. Maybe it's better to rename it to more generic getType()?

[antonkomarev]

Or there could be issues with instantiating correct notification type from the database record?

[taylorotwell]

broadcastType doesn't make any sense. This has nothing to do with broadcasting.

[antonkomarev]

@taylorotwell I understand it. But how this could be implemented then? I want to have same data in database and in broadcasting messages. Create custom driver in application which will include this PR?