[8.x] Add option to release unique job locks before processing

[paras-malhotra]

Based on our discussion at #35042, there may be use cases where it is desirable to release unique job locks before the job starts processing (to avoid race conditions between job completion and unlocking). This PR allows the option to release the unique job lock before processing if the $uniqueUntilStart is set to true like so:

class ExampleJob implements ShouldQueue, ShouldBeUnique
{
    use InteractsWithQueue, Queueable, Dispatchable;

    public $uniqueUntilStart = true;
}

This is backwards-compatible. If the job raises an error, we don't attempt to re-acquire the lock and may be duplicate enqueued. This should be fine as anyone who wants to use this feature would need to make sure that the job is idempotent because a duplicate job dispatch can happen while the job is processing. This is similar to Sidekiq's unique_until start feature.

[paras-malhotra]

@taylorotwell I've added a variable to ensure the lock is released only once in the job lifecycle.

[taylorotwell]

Are we certain that the failed method is called on the same instance so that the variable's value is still in place?

[taylorotwell]

Got rid of property. Added interface:

class SomeJob implements ShouldBeUniqueUntilProcessing
{

}