[8.x] Allow custom names for queued notifications

[miken32]

Since this value shows up in the output for the queue worker, it would be nice to be able to customise it. Previous behaviour remains intact if no displayName() method is set on the Notification instance.

I wasn't able to find any tests for the existing function. Please point me in the right direction if I've missed something, but this seems like very low-level stuff not really needing a test.

[taylorotwell]

Why isn't the class name enough?

[miken32]

The class name is enough for me, as the application programmer, but for people who don't live in the code it's nice to have the option to put something more human readable in the logs. This is already possible with queued jobs, so I expected to be able to do the same with queued notifications.

[driesvints]

We're not adding labels for hacktoberfest after the influx of spam recently.

[miken32]

@taylorotwell just went through figuring this out again and then remembered that I'd done a PR for it already. Is there a reason it was merged and then immediately reverted?

To expand on my previous comment, it's also helpful when finding a problem with hundreds of notifications per minute to be able to add some kind of differentiation to show up in the logs, such as the recipient address for example.