[10.x] Allow failed job providers to be countable

[timacdonald]

When trying to count the number of failed jobs, you have to retrieve all records into memory and do an in-memory count, e.g.,

$jobs = app(FailedJobProviderInterface::class)->all();

$totalFailedJobs = count($jobs);

For applications with thousands or millions of failed jobs...this is not ideal.

Making the 1st party drivers Countable without introducing a new method on the interface.

I've created a dedicated PR for the Dynamo DB driver, as I don't have a means to test it and am hoping someone from the community can help out: #48178

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Side note: kinda liking leaning on the Countable interface here. Not sure if genius or illegal 🚔.