Add missing functions in Dispatcher contract (#22951)

Author: Quentin Burny

src/Illuminate/Contracts/Bus/Dispatcher.php

@@ -28,4 +28,28 @@ public function dispatchNow($command, $handler = null);
      * @return $this
      */
     public function pipeThrough(array $pipes);
+
+    /**
+     * Determine if the given command has a handler.
+     *
+     * @param  mixed  $command
+     * @return bool
+     */
+    public function hasCommandHandler($command);
+
+    /**
+     * Retrieve the handler for a command.
+     *
+     * @param  mixed  $command
+     * @return bool|mixed
+     */
+    public function getCommandHandler($command);
+
+    /**
+     * Map a command to a handler.
+     *
+     * @param  array  $map
+     * @return $this
+     */
+    public function map(array $map);
 }