[8.x] Allow for chains to be added to batches via PendingChain

[Orrison]

This is an alternative to #34612 that uses the actual Bus::chain method rather than a nested array syntax.

It could be used as so:

$batch = Bus::batch([
            new Test1Job(),
            Bus::chain([
                new Test2Job(),
                new Test3Job(),
                function() {
                    // closure
                },
            ]),
        ])->then(function (Batch $batch) {
            Log::info('Test Complete!');
        })->name('Test')->dispatch();

Each job, even each one in the chain including the closure, is given a batchId just as they normally would and the then only fires once all are completed.

Batching adds an amazing functionality of doing something once a bunch of things are completed. But it could be very helpful to include a chain within the "bunch of things" since sometimes certain things have to happen before the other. But it would be really beneficial if that all could be tracked and something is done once they all are completed within the batch.

[taylorotwell]

So... which PR do you prefer? Is this better? If so, how?

[Orrison]

So... which PR do you prefer? Is this better? If so, how?

I think I prefer this one as it seems a little more like a "native Laravel way".

The other PR does introduce a nested array syntax that may be a little confusing while this makes it very clear that you are including a chain within the batch.

[taylorotwell]

Problem with this PR is you're using a Illuminate\Foundation class within an outside component.