[8.x] Check for context method

[avrahamappel]

Currently, in order to send custom context to the logger on a per-exception basis, it is necessary to override the exceptionContext method in App\Exceptions\Handler and check the exception instance. This PR enables the base Handler to detect a context method defined on the exception class which would return an array, keeping the context logic within the exception class.

class InvalidOrderException extends Exception
{
    /**
     * Provide exception context.
     *
     * @return array
     */
     public function context()
     {
         return ['id' => $this->order->id];
     }
}

Once this is merged, I can make a PR to the docs demonstrating this.