[ENQUEUE] Precise phpdoc for static analysis

HI @makasim,

The CommandSubscriberInterface::getSubscribedCommand is currently annotated with string|array return type. Static analysis tool allows to provide a more precise return type. This PR allow PHPStan and Psalm users to have to proper static analysis of the return type when implementing the interface.

I based the array type on the current phpdoc, talking about

     * [
     *   'command' => 'aSubscribedCommand',
     *   'processor' => 'aProcessorName',
     *   'queue' => 'a_client_queue_name',
     *   'prefix_queue' => true,
     *   'exclusive' => true,
     * ]

php-enqueue / enqueue-dev

[ENQUEUE] Precise phpdoc for static analysis #1306