Add check connection function.

[ValikoDorodnov]

Added opportunity to check existing connection in \Bschmitt\Amqp\Request.

For example, if you publish messages in loop and don`t want to create a new connection every time, you may check existing one.

usage example

    public function __construct(
        private \Bschmitt\Amqp\Publisher $publisher
    )
    {
    }

    // usage in methods
   if (!$this->publisher->isConnected()) {
         $this->publisher->connect();
   }