[8.x] Add optional 'exclude' parameter to the PruneCommand

[jochensengier]

This PR adds an optional 'exclude' parameter to the Illuminate\Database\Console\PruneCommand in order to exclude certain models that were automatically detected within the application's app\Models directory.

$schedule->command('model:prune', [
    '--exclude' => [Test::class, Example::class],
])->daily()

Of course, you could argue to remove the Illuminate\Database\Eloquent\Prunable or Illuminate\Database\Eloquent\MassPrunable trait from those models.

But what if you have a base class within the application's app\Models directory, which is implemented by other models and implements one of the above traits?

That base class will not be able to return an \Illuminate\Database\Eloquent\Builder instance, as it does not have a corresponding database table.

Looking forward to any feedback and suggestions regarding this PR.

Thanks!